Zaps and bangs

A reader writes:

Hi, Dan! 

Though I didn't follow all of the details, I did enjoy your writing about electrocution and car batteries.

Do you know the odds of getting electrocuted if one is standing in a wet shower with wet skin using a cordless (battery-powered) sander? I don't know what kind of power I'll need to work on residential showers for hours at a time, but the electric chorded sander I WAS using (until I decided that I'm tired of risking electrocution) says it's a 120 Volt, 10 Amp model. 


There's probably no danger, but there could be some.

Cordless tools all run from low-voltage DC, although the voltage has risen in recent tools that use one or another flavour of rechargeable lithium battery. Higher voltage is better, from the tool-makers' point of view, because a given power from a higher voltage requires less current. This means thinner wires, less beefy switches, and generally speaking a cheaper, lighter tool with the same power.

Cordless tools are also, in general, significantly less powerful than corded versions. It's normal for corded drills and saws and sanders and such to draw peak power of at least several hundred watts. The ten-amp 120-volt rating of the sander you mention makes it a 1200-watt unit (so I presume you're talking about a belt sander, not an orbital one), though it'll only draw that much when it's working hard. You can expect even big heavy cordless tools to have no more than half the power rating of a similar corded tool.

Discovering exactly what that rating is can be difficult, partly because cordless tools can have a larger range between their "spinning freely doing nothing" and "working so hard it's barely turning at full power" power consumption than corded tools do. Mainly, though, cordless power ratings are harder to find because consumers think more watts are always better. So a cordless tool that costs three times as much as the wall-powered version, yet has a third the power rating, won't sell well, unless the manufacturer conceals that latter number.

I'm telling you all this just to explain my original wishy-washy "possibly dangerous" statement. If you're using a 12V tool then you probably won't be able to do yourself any electrical harm with it, even if you smash the thing on the wall until it breaks and then smack yourself in the chest with the pointy bits.

A 36-volt tool, on the other hand, is edging up toward the kind of voltage that actually can harm you, if only indirectly. (Direct harm: Current through your heart stops it, you die. Indirect harm: Current through some other part of your body causes you to spasm and dig a tool into yourself, fall off a ladder, flop out of the shower recess and smack your head on the toilet, et cetera. This sort of secondary injury following a non-fatal shock is a lot more common than injury or death caused directly by electricity.)

In the real world, even crappy bargain-basement cordless tools have enough plastic between you and the wiring battery terminals that no matter what voltage they run at, you pretty much have to make a specific and deliberate project out of killing yourself with one. Working in a wet environment is still dangerous, but only because it makes it easier to slip and then drill, saw or sand yourself instead of the workpiece.

Brand-name tools are generally safer still, and adding water to the situation may ruin the tool but is unlikely to hurt the user. Even the commonly-recognised-as-lethal "dropping a hair-dryer into your bath" situation is actually not terribly likely to kill you, though I don't recommend you try your luck.

If it's possible to electrocute yourself with cordless-tool gear in any way at all, here is I think your best chance of doing it without specifically running wires from the inside of the tool to nails driven into your chest. There are plenty of battery designs with exposed terminals of one kind or another, so suppose you eject the battery from the tool by accident, and then somehow grab that battery with both, wet, hands, so positive is touching one hand and negative is touching the other.

Even then, the resistance of human skin is way up in the tens of thousands of ohms - I found the resistance between two closely-spaced points on my tongue to be 70,000 ohms. So even with a 36-volt battery it'd be surprising if one whole milliamp managed to flow across your chest, and not all of that would go through your heart. I think you'd be an easy order of magnitude away from enough current through the heart for there to be any risk at all.

(I'm sorry to say that I'm not about to conduct heart-stopping experiments on myself. I have, however, previously zapped my arm for science.)

If both of your hands had bleeding cuts on them then 36 volts might be enough to at least give you a shock you could feel and it might have cardiac consequences, but this is really pushing it. And any sort of work gloves not made of chain-mail would erase the risk completely.

And, of course, back in the real world it continues to be downright difficult to actually touch the positive with one hand and the negative with the other. If you just grabbed both terminals of a 36-volt battery with one wet bleeding skinless lightly-salted hand then it'd sting like a bugger, but once again the only real health risk it'd present would be if the pain startled you enough that you then hurt yourself in some other way.

I won't be surprised if cordless-tool voltages rise further, though. There are already cordless mowers that run from 48-volt packs, for instance. So it's possible that a few years from now there'll be cordless tools running from voltages high enough to pose real electrocution risks.

It'll still be a lot less dangerous than it was in the olden days of corded tools, though, when casings were still commonly made of shiny cast aluminium. Then, the user's life was in the hands of the manufacturers and electricians who're meant to keep earth wires connected, and prevent live wires from touching the tool chassis.

With modern plastic casings and other construction improvements, even a theoretical 96-volt cordless tool is not likely to be an electrocution risk, even if you use it in the rain or, more realistically, get all hot and sweaty while working.

There's a lot of energy in a cordless-tool battery, though, and they definitely can hurt you if that energy is released very quickly because of, say, a short circuit...

...or severe over-charge...

...or physical damage...

The reason why drills and laptops and iPads aren't exploding all over the place is that the naturally excitable personality of lithium-ion technology, in particular, is kept calm by strong casings and protection circuitry ranging from simple fuses to smart current limiting:

If one of your cordless tools manages to puncture the battery of another, though, your life may still become quite exciting.

So I suppose I've allayed your fears of one kind of injury and then given you a new one to worry about.

There's no need to thank me.

Psycho Science is a... sort of... regular feature here. Ask me your science questions, and I'll answer them. Probably.

And then commenters will, I hope, correct at least the most obvious flaws in my answer.

On the Domestication of Car Stereos

A reader writes:

I have been rereading some of your stuff concerning speaker building lately and have decided to possibly make it a hobby. A mate and I have a bunch of car audio stuff we pulled out of his car that we would like to turn into a garage/shed powered speaker box. I have been looking on the web and there seems to be about a million different opinions as to various aspects of the build including:

* if we can use the car amp
* if a head deck needs to be involved
* power supply – some say even an old computer PSU can be used some say that the current requirements would be better suited to a car battery with a charger attached (seems a little cumbersome)

So I thought I would send a quick email to great and powerful Tech Master who started me down on this road for some clarification. Help?


Ludicrous car speakers
(Image source: Flickr user Nazly)
Just lever 'em out, and shovel 'em into the house!

Yes, you can make perfectly good household speakers out of car-audio components. Actually, car radio tuners can be better than the home-hi-fi kind, because the extremely variable reception conditions for mobile radio, and the lousy antennas they have to use, mean even quite cheap car stereos often have very good RF sections.

The problem with powering car amplifiers at home is, as you've already discovered, that they can want a lot of current at 12 volts.

First, definitions: You standard "car stereo" has one "head unit" in the dashboard, which combines all of the signal sources - tuner, cassette deck, CD player, line-in socket, memory-card socket for MP3s, whatever - and all of the amplifiers, in one box. These days it probably has four outputs, for two front and two rear speakers. Take such an all-in-one head unit and some speakers and a twelve-volt power supply and hook them all up at home and they'll work the same as they did in the car.

Fancier car audio systems have amplifiers separate from the head unit, and may have multiple head components as well, possibly including a separate satellite-navigation screen, DVD player, et cetera.

A separate car-audio amplifier is basically the same as a separate home hi-fi amplifier: It accepts line-level input, and amplifies it to drive speakers. The input has to come from other components. Fancy car-audio head units may have no amplifiers of their own at all. Again, though, you can run the whole system spread out on a table at home, if you want to and have a suitable power supply.

Standard cheap car-audio amplifiers, like the ones that're built into low-cost one-piece head units, have a maximum output voltage no higher than the 12 volts they run from - 13.8 volts, actually, when the engine's running. Car speakers have a nominal impedance of four ohms - home hi-fi speakers are usually nominally eight ohms - so you can use Ohm's Law to figure out the total possible output current. Ohm's Law says current equals voltage divided by resistance (I = V/R), 13.8 volts divided by four ohms gives 3.45 amps, so that's the most that such an amplifier can output per channel. Each channel is basically its own separate amplifier.

(The "nominal" in "nominal impedance" just means that that's the approximate impedance the speaker presents if you run DC electricity through it. The actual impedance varies quite widely depending on the frequency of the incoming alternating-current music signal, but overall it'll be close enough to the rated nominal impedance for rough-calculation purposes.)

13.8 volts times 3.45 amps is 47.61 watts; an amp with four output channels could therefore output 190 watts, which is more than enough to make the inside of your car very loud, even given the not-so-great efficiency of a lot of car speakers.

(Car-audio equipment of all sorts often has outrageously high "peak music power output" numbers printed on it; this problem may be even worse in car audio than it is in cheap home hi-fi equipment.)

Realistically, to avoid hideous distortion from winding the amp up all the way and to also take into account the rather-less-than-100% efficiency of all amplifiers, a ballpark figure of 25 watts of input power per channel, at the 12 volts you'll probably be running it from at home, is likely to be about the real-world maximum for a basic car-audio head unit. Call it 24 watts for a nice round number of two amps, at 12 volts, per output channel.

More powerful car amplifiers step up their input voltage so they can deliver more volts of output, and the sky's the limit for those. But you can run any amp expecting 12-volt power input from any 12-volt source. As long as you keep the volume low enough that the power supply isn't overloaded, you can probably even run some preposterous multi-kilowatt boom-car amp, at very low volume settings, from a one-amp plugpack.

(This is related to the reason why it's not dangerous to touch both terminals of a car battery that can deliver hundreds of amps into a load with a low enough resistance, like the starter motor. Your body has a far higher resistance, so far less current flows. There are also 12V power supplies with very low output ratings, like say the plugpack for an ancient calculator; you might be able to blow one of those up by just turning on a 12V amp connected to it. You also might be able to damage a 12V amp by plugging an old-style heavyweight unregulated linear power supply into it, because those deliver root-two times their rated voltage when they're unloaded, and seventeen volts might be too much for the amp. All care, no responsibility. Et cetera.)

A PC power supply actually is a pretty good 12V source for running car audio gear at home. It'll be able to deliver a decent number of amps at 12 volts (the yellow wires coming out of standard PC PSUs are +12V), and it should also deal elegantly with overload, and just shut down if you ask for too much current. Modern PSUs may have split 12V rails and other complexities, but an old one out of a superannuated Pentium II box should do nicely for most purposes. Since most home-audio listening happens at only a few watts per channel, unless you want Party Volume of want to hear the music over your power tools, this option should be fine.

A battery charger and car battery will let you run a car amp at maximum power - well, until the battery goes flat because the charger can't keep up with the amplifier load, at any rate. It is as you say not a very elegant solution, though, and the charger may get confused when you turn the amp up and it suddenly sees a strangely high load. And the output from cheap car chargers can be really filthy, noise-wise, too. Car amps generally deal very well with distortion in their incoming power, but you may still hear a whine or ticking noise.

You can also, however, plug car speakers into a home hi-fi amplifier. A given volume setting will give you more noise from a four-ohm speaker than from the eight-ohm speakers home amplifiers expect, but pretty much any amp should work fine with four-ohm-nominal speakers; some home speakers actually have six- or four-ohm nominal impedance. Any old mini-system hi-fi amp will do, too, you don't need a fancy expensive one.

Another way car-audio enthusiasts get more noise out of an amplifier is by wiring multiple four-ohm-nominal speaker drivers in parallel. Two 4-ohm drivers in parallel give you a nominal-2-ohm speaker, three in parallel give 1.33 ohms, four in parallel give one ohm, and so on. You have to be careful connecting speakers with very low nominal impedance to most amplifiers; it usually won't cause a problem if keep the volume setting very low, but one-ohm or lower speaker arrays may look like a dead short to any amp that isn't designed, as some car amps are, to drive them.

You can also get small 12V amplifiers intended for use in home and mobile applications, which are descendants of the popular, and distinctive-looking, Sonic Impact T-Amp:

T-amp and speaker drivers
(Image source: Flickr user animakitty)

The modern ones are easy to spot; they all have extruded aluminium cases, often anodised a cheerful colour, and a few chunky controls on the front:

Lepai amplifier
(Image source: Flickr user

These little amps may be sonically superior to cheap car-audio amplifiers. The radio tuner in a car stereo may have to be good, but car amps can be quite noisy and distorted, because that's not very noticeable in the lousy acoustic environment of the average car. These little units all use similar class-D amplifier chips, which are good for maybe 15 clean output watts per channel or 25 watts flat out. For full power you have to run them from something with at least a five-amp power rating - so, a car electrical system or computer power supply, or a laptop-power-supply-style 12V PSU, which last is what they come with if you buy one with PSU included. Again, though, you can hook up a smaller 12V plugpack if you like, and just keep the volume low.

(Some of these little amps have a USB socket on the front panel, too. In the cheap ones, only have the power pins will be connected, so you can use that socket to charge most phones or MP3 players, but you can't play MP3s off a thumb drive.)

These little amps may or may not sound any better than a cheap car head unit, and all they are is an amplifier - no tuner, CD player or whatever. But they look a lot better indoors than a bare car head unit, and the going rate for one without a power supply is under $US20 delivered.

As I've mentioned before, you can find these amps by searching for "class D" or "class T" (Tripath's trademarked version of class D). You'll find the very cheapest ones if you just search for 12V amps, though. You can get these same chips on little amp-module circuit boards, too, for hobbyists to install in their own enclosures, like a normal sort of amplifier enclosure or powered speakers. The above search is sorted by price, so finds lots of those little modules before it gets to the assembled amplifiers.

Car speakers are a great choice if you want to play with transmission-line speakers, too. The basic transmission-line design has a single widerange driver at one end of a folded tube; here's one under construction:

Building a transmission-line speaker
(Image source: Flickr user Moisturizing Tranquilizers)

A cheap six-by-nine oval car driver with a separate tweeter and/or midrange on a bridge in the middle of it is an excellent low-cost choice for a speaker like this.

More green ink by e-mail

A reader writes:

Dear Dan
I purchased some modulators from Mr Orchard and had one of the units tested using a machine called a PowerMate that is made in Adelaide.

The result was a 30% reduction in power consumption. The test was done over a 3 month period.

Mr Orchard is way ahead of his time. People just on get it!



The contents of this e-mail is highly confidential and for the intended recipient only and to the e-mail address to which it has been addressed to. It's contents may not be disclosed to or used by any other 3rd party other than this addressee, nor may it be duplicated in any way or format without prior consent by the sender. If received in error, please contact the sender by email quoting the name of the sender and the addressee and delete it from your email server and email client software. The sender does not accept any responsibility for any forms of viruses, spyware or malware. It is the responsibility of the receiver to scan all their incoming e-mails and all attachments that have been sent to them.

First, no, e-mail sent to a stranger is not confidential, and no disclaimer boilerplate at the end can make it so. (I'm not sure what the "commercial" part is supposed to mean, either.)

EMPower Modulator

That aside, I presume you're sincere about your statement about seeing the magical EMPower Modulator doing at least one of the numerous extraordinary things it's meant to, and I will also grant for the sake of argument that the test you saw was not rigged, or performed with a defective power meter. (The "Power Mate" is I think meant to be able to take reactive loads into account; cheap power meters like the ones I write about here cannot fully do this.)

In that case, all I can say to you is the same thing I say to everybody who says they know of some gadget that reduces electricity consumption, or improves fuel economy, or in some other way could save a lot of people a lot of money:

Why is the person who has been selling this thing for so many years or, in many cases including that of Harmonic Products, DECADES, not a billionaire Nobel-Prize winner?

You demonstrate your device informally. You talk journalists and a technical college or two into testing it. With that evidence, you talk serious test labs and/or universities into testing it. And then there you are with your proven invention that, because most of the world's population will want it, is not worth millions of dollars; it's worth billions. Hell, even if an evil corporate conspiracy steals your invention, rips up your patent and robs you of your rightful reward, you will still have greatly bettered the lot of humankind. Provided, of course, that the evil conspiracy doesn't tuck your gadget away in the same vast warehouse where they keep the Ark of the Covenant and the hundred-mile-per-gallon carburettor.

There are hundreds of these things. Fuel savers, power savers, perpetual-motion machines, things that allegedly enhance health or cure deadly diseases by means unknown to science, and of course persons distributing the wisdom of super-advanced aliens via channelling.

All could revolutionise the world, if true. None have ever managed it. They always just sell the gadgets, or tickets to their performances, one at a time to punters like you.

(And, notably, they do not mysteriously vanish when the abovementioned giant corporate Illuminati Freemason conspiracy catches up with them. A lot of these people have been selling the same scam pretty much all their lives, without any repercussions beyond getting serially busted by the government because they keep taking people's money and running.)

The closest these miracle devices and potions get to actual success is when they manage to be bought in quantity by someone who hasn't applied any proper tests to see if they work, or who are just hoping to turn a buck on resale or shares in the company. See the ADE 651 "bomb detector" and its various relatives, for instance, and the whole miserable Firepower saga.

If the EMPower Modulator works, it is a miraculous device, and I use that word advisedly. (The same goes for the pieces of purple aluminium jewellery that Harmonic Products told me protect the wearer from radiation, make beverages take better, make metal on your person invisible to metal detectors unless you intend to do something bad with that metal, et cetera et cetera.)

But apparently Harmonic Products are perfectly happy to frame a lottery ticket and hang it on the wall for visitors to admire.

They say it'd win a billion dollars, if they only cashed it in.

Why haven't they?

UPDATE: Peter replied to me, with the following cogent rebuttal:

Yes the world is flat and the Sun revolves around the earth.
Happy sleepwalking.

Sent from my iPhone

I'm not sure whether he's agreeing with me or not.

(There was no boilerplate confidentiality disclaimer this time. Presumably he's cool with his e-mail being published, provided he sent it from his phone.)

More photons, fewer pixies

A reader writes:

This gravity powered light has been getting a little attention lately. It reminded me of your post on the Gravia light.

So is the GravityLight a sensible design that conforms to the laws of physics, or is it also powered by Pixie Dust?


Gravity Light

There's nothing inherently wrong with the basic idea of converting the energy of a falling mass into electricity. (That's how hydoelectric power stations work, after all.) It may even be possible to do it quite efficiently, and cheaply, on a small scale now.

In the olden days you would have needed to gear up your dynamo a lot from the pulley your falling weight (or flow of stream water) was turning, but today super-powerful magnets are very cheap and so efficient lower-speed dynamos are easier to make. Especially if you're only trying to light one high-intensity LED to a brightness that'll let someone read a book at close range.

The GravityLight people claim thirty minutes of light per lift of the weight, and the tape connecting the weight to the light looks to be, being generous, about 1.5 metres long. They don't say exactly what the weight is, but they do say you can fill the GravityLight weight bag...

GravityLight kit

...with use "anything weighing about 20lbs"; let's again be generous and say you get ten kilos of stuff in there, which is a perfectly liftable weight for a wide range of humans.

So we've got ten kilograms falling 1.5 metres in standard gravitation; that gives us a maximum of 147 joules to play with.

Split over thirty minutes, which is 1800 seconds, that gives us only 0.082 joules per second. That means a power of 0.082 watts, or 82 milliwatts.

A single standard white LED will have spec-sheet power numbers of about 20 milliamps at 3.6 volts, which is 72 milliwatts. It'll work fine - and more efficiently - at rather lower current, though, which is just as well because this falling-weight system is certain to be a long way short of 100% efficient at turning the weight's gravitational potential energy into light.

Even if it's only 50% efficient, though, you've still got 36 milliwatts to play with, which is plenty to light one LED to a useful, though far from room-filling, brightness. Even if you pare off some of the above generous assumptions about weight and fall distance, you'll still be easily above 20 milliwatts, which is also usefully bright.

The GravityLight people say their invention is intended to replace kerosene lanterns, but it's definitely not going to have the room-filling brightness of a kerosene lamp with the wick turned well up. Given the numerous downsides of kerosene lighting, though, and the fact that a lot of poor people probably don't turn their lamps up to max very often, a GravityLight or two could well replace one.

The ridiculous Gravia concept thing had a heavier weight falling a similar distance, for a total of about 271 joules. But the designer idiotically claimed that the thing would have the light output of a forty-watt incandescent bulb for four hours. This was so impossible that it couldn't come anywhere near being done even in Physics Experiment Land, with a perfectly efficient dynamo and a perfectly efficient lamp.

With real-world hardware, in contrast, the GravityLight can work. I'm not totally convinced that for household lighting you wouldn't be better off with a couple of conductive objects a reasonable distance from each other in the galvanic series, some damp earth as an eletrolyte, and a Joule Thief to boost the output to run an LED. That sort of improvised battery can run for a very long time at the very low power a Joule-Thiefed LED requires, and its poor portability doesn't matter if you're using it as a hoursehold light (and is an advantage if you want to avoid your light being stolen...).

But the physics, at least, checks out for the GravityLight.

12 AAs in the magazine, one in the chamber

A reader writes:

I've read that the problem with ray guns is that as an energy delivery system, pieces of high-speed lead propelled by a chemical reaction work much better than photons propelled by battery power.

If you could dump all of the energy out of, say, a AA battery really fast, though, could you get bullet levels of energy out of each battery?


Yes, you could.

Let's presume you're using nickel-metal-hydride AA batteries, which are somewhere between average-rifle-cartridge and average-pistol-cartridge in size. You can get a lot more current out of a NiMH or NiCd rechargeable than an alkaline or carbon-zinc battery, but, as you say, you still can't discharge them nearly fast enough for them to be useful replacements for firearm cartridges.

Even if you don't care whether the battery survives the experience, the biggest bang you can get out of a battery is the feeble "explosion" of a laptop battery. That may give you nasty burns if it happens literally on your lap, and shorted batteries have been responsible for the destruction of quite a few cargo planes, but batteries are no more than firecrackers compared with proper explosive devices.

Never mind that for now, though, let's just look at the energy content.

The most generally useful kind of NiMH cell is the "low self-discharge" type, which unlike the older kind of NiMH, do not go flat in a matter of weeks whether you use them or not. (Low-self-discharge cells are often sold as "pre-charged", or "ready to use".) LSD cells have lower capacity, though, so let's say we're using non-LSD cells with the absolute bleeding edge maximum capacity today available, which is about three amp-hours (3000 milliamp-hours).

1.2 volts (the standard NiMH or NiCd terminal voltage) times three amp-hours gives 3.6 watt-hours. A joule is a watt-second, there are 3600 seconds in an hour, so 3.6 watt-hours is 12,960 joules.

Firearm muzzle energy is often measured in foot-pounds, not joules, but I'll keep it all in SI units here. You also couldn't get the entire capacity of any electrical energy source into your beam or projectile, because no laser or mass-driver is 100% efficient, but I'll handwave that as well.

12,960 joules is a pretty darn respectable chunk of energy, way more than any handgun cartridge can manage. 9mm rounds top out around 500 joules of muzzle energy, .44 Magnum is a couple of thousand joules at most, and even the ludicrous .500 S&W Magnum is only around 4000 joules.

Rifle cartridges that qualify as "high-powered" seldom exceed 4000 joules. You have to start looking at exotic specialised sniper and large-game rounds, or heavy-machine-gun ammunition, before you get above ten thousand joules. The .50 BMG round easily beats 12,000 joules, and the more ludicrous kinds of elephant-gun double rifle roughly equal the battery's energy... you'd bleeding well want them to, for this kind of recoil punishment.

But all of this is, again, just fantasy, because you can't dump the energy out of any kind of battery anywhere near fast enough to make it useful in a gun.

You can, however, dump the energy out of a capacitor in a very short period of time.

The highest-capacity "supercapacitors" can't be discharged in a tiny fraction of a second without damaging them; they're usable in a flashlight or for regenerative braking, but not for one lightning-strike discharge, as in a firearm.

Normal caps certainly can be discharged fast, though.

This cap bank could be used to power some kind of kinetic or ray-gun weapon - but it takes all of those huge beer-can capacitors to hold a mere 11.3 kilojoules, roughly the same as our one AA NiMH cell. The one killing the watermelon above is only 9270 joules, and its caps are huge.

I have one beer-can electrolytic cap of my own; it featured in this...

Ridiculous contraption

...extremely practical assemblage.

If it still had its full original capacity - which it doesn't - then fully charged to its 850-microfarad, 450-volt redline, it would hold 86 joules of energy.

You can get firearm cartridges that are that feeble, or even weaker, and I certainly wouldn't want to be shot with them. But I'd take them over a humble .38 Special or .22 Long Rifle any day.

The miserable performance-per-size of electricity-storing devices is why electromagnetic railguns are of moderate interest to navies...

...but not to armies.

It's also why the only really workable technology for a military laser gun (as opposed to lasers only used to temporarily or permanently blind the enemy) is the chemical laser.

Chemical lasers can be usefully powerful without requiring capacitor banks the size of a house. They are generally very unpleasant to be near, though, because they either run on, or produce, horrible toxic compounds.

Which is why plain old deflagrating gunpowder, propelling a piece of metal down a tube, remains the standard way to do unto others at a distance.

Oh - and if you've never watched Kaboom!, you really ought to.

UPDATE: On the subject of ludicrous electrical things, there's this piece I did on what a AAA battery composed of nothing but electrons would be like.

(It would not be kind to nearby spacetime.)

Psycho Science is a... sort of... regular feature here. Ask me your science questions, and I'll answer them. Probably.

And then commenters will, I hope, correct at least the most obvious flaws in my answer.

In Which I Try Not To Set A Reader's House On Fire

A reader writes:

I've been searching the internet (including your articles) for information on putting together a simple back-up power supply for my central heating system. One that when the power fails (which it will here in Greece when we get a good old storm), I just go down to the boiler room, disconnect the boiler system from the mains and hook it up to a back-up supply (for the two or three hours that it takes to get the utility repair man out of the taverna, up the pole and get my 230v back on line). Automatic systems are all very well, but isn't it nice to know what is actually going on, and also be in charge!

Most of the back-up power systems that I have found on the internet seem to be designed for computer systems (oh, and maybe a fridge). Well when it's wet and cold here and the power goes down, I am more interested in keeping warm than keeping my beer cold (although I do understand the importance of the later) and if my computer doesn't work, well, I still have my Ipod.

The reason I need back-up power is because my oil fired central heating system has a wood burning stove linked to it and must keep the system (in particular both the pump and the system controls) running. The boiler is rated at 140w, the pump 160w, 5 motorised valves 30w and the control box and lamp 70w - 400w in total.

Is there such a back-up supply suitable for my heating system or do I put up with shivering, writing unnecessary e-mails in the dark over a can of cold beer?


Anything involving monkeying with heating systems raises red flags with me, but I'm pretty sure I'm not about to give you advice that will lead to your death. I have, however, been repeatedly demonstrated to have very poor judgement in this regard. (Some friends of ours have officially notified their small children that not everything Daniel says should be accorded the same respect as things said by other adults. They were fine with this, though.)

There may also be some local law that makes this illegal, or requires a licensed electrician to install it, or something; I know nothing of Greek law.

OK, disclaimers over. If you're happy to have a setup that you have to go into the boiler-room to connect, then I think the best option would be an appropriately robust petrol-powered generator. You have to duct the exhaust outside, of course, or set the generator itself up outside. (It might be possible to plumb the generator exhaust into the boiler flue or something, but this could also be another piece of extremely dangerous advice.) Apart from the exhaust issue, though, it'd probably work nicely. Modern generators from the major manufacturers are reliable, quiet and not even all that expensive.

(Generators that serve the purpose of a UPS, cutting in automatically when power fails, are fancier and more expensive. Way more expensive, if you want one that won't give you even half a second of blackout.)

You could probably also use a suitably large off-the-shelf UPS, though, if the tromping into the basement and switching the cables and pulling the starting rope starts to pall. The wattage figures on your heating system's specification stickers are, like most such figures, likely to be over-estimates, so it's possible a quality UPS with as small a rating as 700 volt-amps (which are not quite the same as watts, as I discuss here) could do the job.

The power-rating issue is the same for generators as for UPSes, but I think generators are better at handling the initial "inrush" current when a motor starts. That can be high enough to cause a UPS to beep and shut down, even if the UPS is perfectly able to power the motor if it's already running. This is particularly the case for refrigerators, whose run power is quite low but whose compressors suck a lot of watts for a brief moment when they click on. A UPS trying to power such a motor will therefore work OK if you lose power when the motor's already running, but not if the motor needs to start from UPS power. In the same situation, a similarly marginal generator should just bog down and deliver lower voltage than it's meant to, which is in this case perfectly fine and should let the motor start up with no trouble.

The solution to this whole problem is, of course, to just get a UPS or generator with a higher volt-amp rating, or with a specific high surge capability that it may only be able to deliver for one second, but that'll do. A "home"-model UPS or small generator rated for a genuine 1500 VA (as opposed to the suspiciously high numbers on suspiciously cheap off-brand UPSes) might be adequate; if you turn out to need more than that, and decided to go with a UPS, then you'd have to pay extra for a commercial-market one.

Easy enough to find out what works, of course, if you can get a local dealer to let you borrow likely-looking generators and/or UPSes and try them out. Overload won't actually damage any half-decent UPS or generator; at worst, they'll just complain and shut down.

The real killer for a UPS solution would be run time. Generators can run for as long as you have fuel, of course, but three hours is a long time for a home-or-small-office UPS to be delivering a few hundred watts. Smaller UPSes may even overheat and die in such a situation.

If we presume the constant draw is, say, 400 watts, then that for three hours is 1200 watt-hours, and the battery-to-UPS-to-appliance chain is not 100% efficient, so you'd need more than 1300 watt-hours of batteries to get it done. The capacity of the standard little sealed-lead-acid brick batteries in small UPSes is maybe 90 watt-hours. Less, actually, if you don't want them to die young; standard lead-acid batteries don't like being run flat.

Commercial UPSes can usually be had with extended batteries, but regular readers will know that I recommend just hooking up some car batteries instead. The very cheapest car batteries are still good for 240 watt-hours or more, so it'd be inelegant but feasible to build an array out of them that could meet your needs.

Drop some extra dough on quality "deep-cycle" batteries that are actually meant to do this kind of job and you can easily get well over a thousand watt-hours from one 12V battery that two normal humans can probably move. Graduate to proper industrial batteries that only strongman contestants can move by themselves and you probably won't actually get a whole lot more capacity per kilogram, but probably will get a setup that'll work for many, many years with no more maintenance than an occasional distilled-water top-up. Industrial batteries and a commercial UPS should actually easily outlive a generator.

The smallest batteries in the current Trojan Industrial Line, for instance (PDF here), are six-volt with a 355-amp-hour rating even if you're running them flat over only five hours; two of those in series will give you 4000 watt-hours at 12 volts even into quite a large load.

But, again, none of this is necessary if a generator's acceptable to you. Since you specifically asked for a system that requires you to switch it over manually, a standard, quite inexpensive pull-start generator looks like just what you want.

(I invite commenters to point out the many ways in which I have, in the above, unknowingly endangered Bill's life.)

From crowbar to glowbar

Glowing crowbar

A reader alerted me to this video (which has embedding turned off - click through to YouTube to watch it). His subject line was "Progress in the field of vaporizing crowbars".

(He found it via Hack A Day, where I would eventually have seen it myself, but I've got 234 unread articles in the Hack A Day feed, so it could be a while before I get to it.)

In this video, there is a man.

The man has made a transformer.

Monstrous transformer winding

The transformer's secondary winding looks like a suspension component from a large four-wheel-drive vehicle.

Light-duty hookup wire

You could moor a ship with this stuff.

There is, I must warn you, a certain amount of profanity in the video.

Crowbar sparks

I think it is entirely justified, given that in this gentleman's estimation thirty thousand amps does not qualify as "serious amps".

Screwdriver abuse

Bendy hot screwdriver

"I've got a screwdriver what goes round corners, now!"

Giant variac

I note that the transformer also seems to be running from a variac that makes my 500VA one look very, very inadequate.


The immense current capacity of the transformer causes anything shorting the outputs via small contact points to instantly lose those contact points in a most impressive explosion of sparks. But since the transformer in its present configuration tops out at only about four volts open-circuit, the hazard it (as opposed to its mains-voltage power supply) poses to its operator is only one of burning yourself on hot conductive objects, not electrocution.

"Don't touch anything electrified" is one of those general rules of thumb like "don't put metal in a microwave oven" that are easy to explain to people, but which do not actually apply in every situation.

I still wouldn't want to walk around this guy's house blindfolded, though.



Alarm clocks are awful. Get one of these instead.

Even if your alarm clock is one of those Zen alarm clocks with melodious metal chimes, or it's your phone playing New Age music at gradually increasing volume, an alarm clock is still not offering you anything. It's just invading your rest and causing you to start your day with a little slap of sadness and irritation, arguably made even worse by the snooze button's empty gift of a few more minutes of half-sleep. Which you'll probably only spend trying to integrate your interrupted dreams with wakefulness.

(It is, at least, pleasant when I realise for the umpteenth time that I am not, in fact, still in high school.)

Or you may just lie there, living in fear of the return of the cursed alarm.

I shudder to think how much human misery those millions of little morning insults have added up to, over the centuries since humans first invented a water clock that would make a noise at a particular time.

Presuming you can't just rearrange your life so it doesn't matter when you get up, the best option in the pursuit of timed wakefulness is, clearly, a butler. A butler who brings you a cup of tea, even as he murmurs his apology for the regrettable necessity that you be conscious.

(The celebrated Stephen Fry alarm clock seems to no longer be in production, and it would also appear that more people paid for them than actually, strictly speaking, received a product in return. One can only hope that the Bible-verse version of the same product is similarly defunct. I would very much like to see a combination of the two.)

Failing that, what you want is a teasmade.


This one's mine. It's not a particularly elegant or collectible example, but it does the job.

The modern teasmade - the term has become a genericised trademark, in Britain at least - is essentially an electric kettle controlled by an alarm clock. When the alarm time is reached, the kettle element turns on, and a few minutes later boiling water is delivered to the tea-leaves.

Or to anything else you put in the little teapot, for that matter. If you want alarm-clock instant soup or Bovril or anything else you can prepare by putting it in a pot and pouring boiling water on it, you can have that too. (Might be a bit of a challenge eating soup out of the little teapot, though, if there are bits in it too big to exit the spout. You could also make a hot-milk-based beverage in a teasmade. But possibly only once.)

Most Automatic Food Machines, especially the ones that look like the coolest thing in the universe, have problems. They don't work in the first place, or they work only for a little while without unfeasible amounts of maintenance, or they're uncleanable, or their sole desire is to maim or murder their operator.

For every good gadget like the blade meat tenderiser or Aeropress coffee maker, there are a dozen crappy As Seen On TV wastes of time and money.

(There's also a small sub-category of wonder food gadgets that can only work by breaking laws of physics.)

A teasmade is not like that.

It is easy for a tired person to set up of an evening, it does what it's supposed to do without fuss, it has no moving parts except the control buttons, and its cleaning requirements are close to zero.

(Classically, you are never meant to do anything more than rinse a teapot; the accumulating tannin stains are supposed to make the tea taste better, though I'm pretty sure that claim doesn't stand up to double-blind testing. If you've got unusually hard water or use a teasmade for a long time then you may also need to clean lime-scale out of the boiling vessel and tube, but just running the teasmade with water and vinegar in the boiler should take care of this.)

Antique brass teasmade

The earliest alarm-clock tea-makers were created in the late nineteenth century. They were shiny and clockwork, with alcohol burners and a match-striker or other similarly implausible mechanism to light the burner when the alarm went off. These devices were less of a threat to life and limb than one might imagine, but were still less than entirely convenient to operate, and also rather expensive. And, to be fair, if there's one thing that'll wake you up even more effectively than a nice cup of tea, it's a fire on your bedside table.

The modern teasmade is electric, safe and reliable, and quite cheap. I bought mine used on eBay in 2003, and it cost me only $AU37.85 delivered. That was an unusually good deal - one just like it is on as I write this, for $AU90 plus delivery - but working and pretty-safe-looking used teasmades routinely sell for well under $US100 delivered, and you can get a brand new one for less than 60 UK pounds delivered within the UK, $AU160-odd delivered to Australia, or around $US165 delivered to the States. Or less, if you buy the version with no radio, of which more shortly.

And yes, American buyers are likely to have voltage problems, because the teasmade is largely unknown outside the UK and as far as I know nobody makes a 110-volt one. More about that in the "buying one" section, below.

How it works


As you can see in the above picture, the modern teasmade really is essentially just a combination of an alarm clock - often, as in mine, a clock-radio - and an electric jug. It's quite easy to use.

The alarm clock in my teasmade works in the same way as every cheap plastic clock-radio. You set the time, you set the alarm, you tune the radio, and you select how you want the thing to wake you up.

Teasmade controls

In addition to the standard clock-radio options of an awful alarm noise or a tinny radio, though, my teasmade lets you select "tea" alone. You will then be awakened by the sound of boiling water, and the smell of a mildly caffeinated beverage.

My teasmade is rated at 600 watts at 240 volts, which is on the low side by electric-jug standards; here in Australia our mains electricity is a nominal 230 volts and a usual actual 240, so electric jugs with a power rating of 2000 watts or more are common.

My teasmade's water capacity is only about 650 millilitres - that's about 2.6 metric cups. Or a couple of good-sized mugs, or more than three dainty little teacups. The 600-watt heater takes about seven minutes to boil this full capacity; proportionally less if you don't fill it completely. You should of course take account of this boiling delay when setting the alarm time.

The initial heating process is quiet; it'd probably wake me up if I were sleeping without earplugs, since I'm a pretty light sleeper, but most people would sleep through it. The part where the boiling water is transferred to the teapot, though, is quite dramatically loud, and should be an effective alarm for most people all by itself.

The reason for the noise is the way in which my teasmade, like pretty much all others, transfers the boiling water from kettle to teapot. When you put the filling cap back on the boiling vessel, the boiler is sealed except for a metal tube that goes almost to the bottom of the vessel, and arches over to point at the middle of the lid of the...

Teasmade teapot

...distinctive hole-topped little teapot. (If you find a junk-shop teapot that looks like this, you now know where it came from.)

When the water boils, the pressure of the steam pushes the water through the tube and into the pot. It takes no more than ten seconds for my teasmade to transfer a full pot-worth of water through the rather narrow tube. Hence the noise. When the reservoir's empty, it heats above the boiling point of water and a thermostat shuts off the heater. (There's also a switch that disables the heating element if the teapot isn't in place, so night-time absent-mindedness will not result in an unconfined spray of boiling water the next morning.)

If you need more of an alarm to wake you up then, ideally, you'd be able to set the horrible alarm noise or irritating radio station of your choice to go off when the water transfers, or even after the tea's had a few minutes to steep. But my teasmade can't do that; the alarm/radio goes off when the
heating element turns on, at which point a single cup of tea is at least five minutes away, and a full pot is ten.

Some teasmades have more sophisticated alarm settings, so the alarm can go off when the boiling is completed, not when it starts:

OK, that alarm takes us straight back into the Land of Horrible Awakenings. But at least there is tea.

Going along with its cheap-clock-radio nature, my teasmade has no backup battery, and reverts to that good old flashing "12:00" and no memory of previous settings if there's even a momentary power cut. You can solve this problem by running the teasmade, and for convenience also your bedside lamp, from a small uninterruptible power supply. A pretty beefy UPS will even be able to run the tea-making element; a cheap one won't be able to do that, but will at least ensure continuity of timekeeping if the element doesn't try to click on from UPS power.

I'm hardly an authority on teasmades, though; there are a lot of different models, even if you disregard the pre-electric W. Heath Robinson versions.

Goblin<br />
(Image source: Flickr user James Mooney)

Here's a Goblin with a removable boiler, as well as teapot, presumably for ease of filling.

Goblin<br />
(Image source: Flickr user James Mooney)

TEA NOW<br />
(Image source: Flickr user Martin Deutsch)

I wish mine had a TEA NOW button.

Teasmade with<br />
(Image source: Flickr user leo.j.turner)

Another picture of the same model of teasmade, or at least one with the same buttons. It appears to have won an award. Good for it.

Teasmade with<br />
slightly lopsided lamp
(Image source: Flickr user MarkyBon)

This one just screams Fawlty Towers.

Assorted<br />
(Image source: Flickr user gruntzooki, a.k.a. Cory Doctorow)

Another just like it, and some others, on display in the London Science Museum. It's apparently circa 1945.

The integrated lamps can be rather nice:

Science<br />
Museum teasmade display
(Image source: Flickr user ebbandflo_pomomama)

Teasmade with top<br />
(Image source: Flickr user Simon Harriyott)

This one looks as if it ought to be mystifying Jacques Tati in Play Time.

Buying one

A simple search for "teasmade" (which may or may not correctly geo-target to your country; here the same search is on eBay UK, and here it is on gets 20 relevant hits on as I write this, plus a few isolated teapots and a Bjork remix with "Teasmade" in its title.

There are some decent deals there, but I probably got my teasmade so very cheaply - under $AU40 delivered - because it was described as "Alarm clock/radio with teapot -RARE", which barely describes it and is almost impossible to search eBay for. I've no idea how I ever found it.

Even if all you throw into the eBay search box is some generic "tea maker" sorts of terms, it's pretty much impossible to filter out umpteen ordinary electric jugs, teapots with infusers in them, teapot-shaped kitchen timers and so on. Here's my best effort at making such a search across the whole of with possible geo-targeting to other eBay sites; if that doesn't work, here's one for and here's one for

The easiest way to get a teasmade today is to just buy one new. For a while I think this may have been impossible unless you found a dealer with "new old stock", but now it's quite easy to buy a Swan teasmade online.

Swan teasmade

You may or may not care for the Swan's magic-lantern styling and LCD analogue clock, but on the plus side, you know the appliance hasn't been sitting in someone's garage for fifty years, maturing into a truly world-class fire hazard.

The Swan teasmades list on their site for £79.99 (about $US128 or $AU124, as I write this) ex delivery. They don't ship outside the UK, though.

The Swan teasmade is also on sale at this Union-Jack-waistcoat of a site, which is very excited to announce that the "Teasmade Classic is now £48.99 and the Radio Teasmade is now £69.99!". But they, also, only ship within the UK.

There are Swan teasmades on eBay; a US buyer could get the basic no-radio model for $US132.05 plus a mildly suspicious only $US6.00 for shipping, and an Australian shopper could get the same model for $US142.05 delivered.

That's not cheap, but at least the Aussie shopper would only need a plug adapter to connect a UK-sourced teasmade to Australian mains power.

(Until quite recently, it was normal for UK appliances to come with a power cable that terminated in bare wires, because the UK contained an incompatible mixture of the old BS 546 and new - in the sense of "after World War II" - BS 1363 wiring and plug standards. You had to buy a plug separately and screw it onto the cable yourself, or get someone in the shop to do it for you if you were a wuss. Nowadays BS 1363 is dominant enough that I think pretty much all UK appliances come with a BS 1363 plug moulded onto the end of the cable. Chopping that plug off and replacing it with one to suit your own country's mains, so you don't have to use an adapter, is unwise if you don't know what you're doing, but is legal in most countries.)

If you live in the USA, Canada or some other 110-to-120-volt country, though, you have a problem. Some teasmades wired for 110V are alleged to exist, and converting one wouldn't be an insoluble problem for an electronics hobbyist or repair-person, but you ain't gonna get one off the shelf.

So there's nothing stopping someone in the Americas from buying a teasmade from the UK or Australia, but it'll be the wrong voltage and you'll have to run it from a quite beefy step-up transformer.

Your beefy step-up transformer will very probably be an autotransformer, and very probably come with a piece of paper listing a wide variety of devices they strongly recommend you never plug into it on account of autotransformers' poor isolation qualities. A teasmade is likely to fall into at least two of those forbidden categories.

That said, two-pin sockets and cheater plugs do not yet seem to have killed most of the American population, and those are more straightforwardly dangerous than a teasmade running from a step-up transformer. Modern teasmades also have no exposed metalwork, so you're not really living too dangerously if you use one from a step-up transformer. I'd do it. But I am known for making poor decisions.

You can also get fully electrically isolated step-up transformers; they're more expensive, but solve the safety problems. And if your American house has a 240-volt circuit for a clothes-dryer or other high-powered electrical appliance, you can just plug the teasmade into that. (Running an extension cord from the one 240V outlet in the bathroom all the way to your bedroom may negate the safety benefit.)

The frequency of US 240V will be 60Hz instead of the 50Hz the teasmade expects, which will cause old-style electric clocks to run six-fifths as fast as they should. If you get an old teasmade with an analogue clock then there's a good chance it depends on the mains frequency to keep time, and will thus be essentially useless from the wrong frequency. You could run it from a frequency converter, but by now we're getting well out into the crazy-weed.

As long as there's no mains-synchronous clock in your teasmade, a different mains frequency shouldn't be a problem. A newer teasmade with a digital clock will very probably have a quartz oscillator that's immune to mains frequency changes.


By this point, American shoppers intrigued by the teasmade idea but disinclined to subscribe to British Appliance-Fancier Monthly will probably be thinking there must be a simpler way to do this. I mean, you could just plug an electric kettle into a timer switch and get something approximating the same functionality.

It occurs to me that if you get a coffee-maker that has a timer function, put tea leaves in it in place of coffee (and, if you want to get fancy, also replace the paper filter with a mesh strainer screen), you could get very close to a teasmade's functionality without all of the international-voltage bother.

The design of the typical modern no-moving-parts bubble-pump coffee-maker (which, incidentally, uses as a pump the same sort of device that propels a pop-pop boat)... not ideally suited to making tea, but it'd more or less get the job done. A coffee-maker may not quite make Tea According To Orwell, but I'd drink it.

(Oh, and cheap drip coffee makers' primary purpose appears to be to make coffee snobs almost as apoplectic as percolators do, but a teasmade actually makes pretty close to optimal tea. It doesn't pre-heat the teapot, which is a mark against it, but it does deliver really scalding water onto the tea leaves, which is generally agreed to be Correct. Coffee benefits from being made with less-than-boiling water; tea does not. The fact that water boils at only 174°F at 20,000 feet {79°C at 6100 metres} is clearly a far greater problem for British mountain-climbers than any piffling shortage of oxygen.)

Cheap coffee-makers with timers require you to reset the timer every night, because they can't tell whether there's already coffee in the carafe or not, and want to avoid disasters that a forgotten full carafe could cause the next morning. (Leaving the water reservoir empty shouldn't be a problem, though, because even the very cheapest of coffee-makers should have a reliable overheat cut-off.)

You'd think you could get a coffee-making teasmade-analogue with an actual alarm-clock brewing function for a reasonable price, but I'm not sure if you can. I think you can do it with expensive plumbed-in models like this one, but few mere counter-top models seem to have such a feature.

I think this inexpensive Black & Decker model may qualify, though. I managed to find a manual for it online and it does seem to have a repeating alarm-coffee function. If you definitely know of such a thing, please do tell us all about it in the comments.

(On the subject of different approaches to the problem, check this out. Once again, some concept designer has supported my previously-expressed opinion of the breed, in this case by reinventing the teasmade and making it much, much worse. Apart from apparently being carefully designed to set itself on fire from the middle out, this concept design expects you to, first thing in the morning, drink tea out of a hemispherical E-Z Spill(TM) cup with no handle. This design also requires you to put a tea bag in cold water the night before and let it sit there for hours before the water is heated, which I can only presume will cause the ghost of Queen Victoria to manifest, reach into your chest and crush your worthless heart.)