Perhaps I'll use it as a doorbell

If you had to name one electrical component that just shouts "mad scientist", the knife switch would be that component.

(I'm not counting the Jacob's Ladder as a "component", here.)

Connecting lightning to your not-yet-animated monster, activating your death ray, powering up the time machine; all jobs for a big old two-blade knife switch.

Knife switches have plenty of actual practical uses in the real world. Even small ones can switch very high current, their position is obvious at a glance, and they can put up with a lot of abuse. They're obviously not a great choice for high-voltage switching, but they'll usually actually do that very well too - you just have to stay away from the live bits.

(Knife switches made for really high-voltage operation often have special spring-loaded doodads that stay connected as you raise the knife-bar, then snap up very quickly. Their purpose is to break the contact very rapidly, so you don't pull an arc between the terminals.)

So naturally I had to get one. And not one of the little plastic science-classroom versions with binding posts or spring terminals; I wanted something beefy, as were and still are used to isolate radio gear from the big lightning-attracting antenna outside. A knife switch also makes a dandy automotive battery isolator, but I didn't want one of those, either.

After a year or two of e-mails from my saved eBay search, I found just the thing.

Knife switch - both blades up

This handsome object cost me $AU28.11 delivered, which I thought might have been a bit too much, until it arrived. I now realise I got a bargain. This thing's way cooler than I expected it to be.

All of the terminals and contacts work OK; a couple of the hefty terminal screws were seized and remain tight after cleaning and oiling, but this is a perfectly functional piece of gear.

The Bakelite-slab base is only about 14 centimetres square (5.5 inches), but the whole assembly weighs about 1.86 kilos (4.1 pounds). And it's surprisingly complicated.

Your standard two-blade knife switch is simple enough. It's either a dual-pole, single-throw, or a dual-pole, dual-throw (if you don't know what this means, check out the Wikipedia article on switches).

This thing, in comparison, is a freakin' logic puzzle.

It's got six terminals, and two separately hinged - but electrically connected - blades. The worn (and now lightly polished!) wooden handle is in two parts, too, one for each blade. But the two handle parts form a rebate joint.

Knife switch - one blade up

This makes it possible to have both blades down, both blades up, or only the left blade up. But, because of the rebate joint, you can't have the right blade up and not the left.

Knife switch - both blades down

Let's number the terminals clockwise from the one at the bottom right of this picture. So the one to its left is terminal 2, terminal 3 is the one on the back connected to the bases of the blades, and so on to number 6, which is partly obscured by the wooden handle in the above picture. Pay attention, there will be a test.

With both blades up, terminals 1, 2 and 6 are connected to nothing, and terminals 3, 4 and 5 are connected to each other.

With the right blade down and the left blade up, terminals 1, 2, 4 and 5 are disconnected, while 3 is connected to 6.

With both blades down, terminals 1, 3 and 6 are connected to each other, and terminals 2 and 4 are connected to each other; only terminal 5 is no longer connected to anything.

(If you can't quite see how that is the case, note that the middle section of the left blade, the lower one in the above picture, has a copper sleeve around it that's insulated from the blade itself. When that blade's down, the sleeve connects terminal 2 to terminal 4, but not to the blade itself.)

Oh, and terminals 1 and 6 are connected to the blade contacts via a couple of bits of might-perhaps-be-fuse-wire-but-probably-isn't. So you could easily connect either or both of them to some other part of the assembly, if you wanted.

(Does anybody know of a piece of software that'll take a description like this - "in state A, these parts are connected, in state B, the situation changes to this", et cetera - and will then draw you a diagram? I started drawing it out by hand in a flowcharting/circuit-diagram program, but then realised I had no idea how to draw these crazy ganged switches.)

The baseplate bears a little oval plaque that says:


(It just occurred to me that the switch could easily have been used for switching railway signals of some sort. The rebated handle interlock could be for something like preventing green lights for both directions on one line.)

I actually will use this switch as a switch, from time to time. But when it's not in use, I think I'll hang it on the wall somewhere.

15 Responses to “Perhaps I'll use it as a doorbell”

  1. Daniel Rutter Says:

    You are so lucky!

    I know!

    (Anne totally does not get this. I'll make sure she reads this, to see all us dorks cooing over this thing like 1950s housewives at a Tupperware party.)

    I passed up (or didn't bid enough to buy) several other switches before I finally found this one. The basic antenna-isolator type of knife switch comes up quite often on eBay, and sometimes there are monstrous old things with porcelain handles and no baseplate, because they used to be anchor-bolted to a concrete wall.

    My only question is: Does it still smell like it should?

    It pretty much just smells of kerosene at the moment, because I spritzed it with WD-40 to loosen the crud of decades and get the terminal screws moving again. Now everything moves freely and makes proper contact, and it's also now possible to handle the thing without having to wash your hands afterwards.

    I was pleased, however, to be able to apply the smell-your-thumb test to the base to verify that it was Bakelite.

    That test goes like this:

    1: Vigorously rub some part of the object you think might be Bakelite with the pad of your thumb. You want enough friction that your thumb becomes uncomfortably warm.

    2: Quickly smell your thumb.

    If, for a second or two, there's a distinct smell of phenolic resin - which is kind of hard to describe, but because phenolic is still used for circuit boards anybody who's encountered some hot electronics will know it - then the object is Bakelite.

    This test is a good one because it doesn't harm the thing you're testing - well, not unless it's some fragile little hair clip or something, and you rub so hard that you snap it.

    (There are a variety of other Bakelite tests, the dumbest of which is the "red-hot needle" one, in which you actually burn a little hole in the item you're testing. To be fair, that test will probably also tell you if the item is made of celluloid instead of Bakelite, because the needle will then probably set it on fire. Celluloid burns real good.)

  2. OJW Says:

    Why not just draw the circuit? isn't it something like this:

  3. Simulant5 Says:

    You are so lucky! My only question is: Does it still smell like it should?

  4. Daniel Rutter Says:

    Why not just draw the circuit?

    I see what you did there :-).

    I think you're right, and I made this too difficult for myself by trying to fit this unusual arrangement into a conventional circuit diagram, when you can just sketch it out and put "NOTE: Right blade can only be up if left blade is also up" next to it.

    I think your diagram isn't quite right, though, because it doesn't show the right blade connecting 5 as well as 4 to 3 when it's up, and also shows the left blade connecting 2 to 5 when it's down via the sleeve conductor doodad; that should be 2 to 4, and it should be clear that this is a separate path from the 1-to-3 connection that the left blade also makes when down.

    I think you can fix this with another note or two and possibly also by including the same terminal in more than one place on the diagram, but my brain is seizing up now.

    You could use this thing as an interview question :-).

  5. robzy Says:

    Actually, I think that OJW's diagram is correct except for one small mistake - terminal 4 and terminal 5 should be swapped around.


  6. robzy Says:

    On closer inspection of the pictures I think I was wrong, OJW's diagram is correct without any modifications.

    Except for the fact that it doesn't explain the rebate feature.


  7. OJW Says:

    @robzy just noticed the upper contact is in 2 parts, so it can connect 4 and 5 together (this matches Dan's text description).

    So the diagram does get a bit bigger. Maybe something like

  8. Stoneshop Says:

    Ahhhmmmm, the smell of bakelite. Yum.

    One of the motorcycles I own was built in the Socialist Worker's Paradise that was the German Democratic Republic, by the Motorradwerke Zschopau, MZ for short, in 1981. And of course, like the Trabant and the Praktica, it looked 20 years older than it actually was, with technology to match. Which is not to say that it didn't work fine, but the 6V electric systems the MZ engineers had bestowed on it was somewhat lacking, and consequently riding at night on unlit and otherwise deserted roads a little more exciting than one would like.

    So I ordered a 12V conversion kit. To be mounted in place of the original generator.

    A motorcycle's generator is often mounted on one end of the crankshaft, and when it is, it's often enclosed in the crankcase. This machine being a two-stroke it doesn't have oil present there, and when I opened up the generator compartment a most concentrated bakelite odour wafted out. Did I mention that the crankcase gets hot? That was twenty years of bakelite gassing out its volatile components into a sealed compartment.

  9. Jono4174 Says: looks correct to me

    Here is another way to show such an arrangement. I have seen it used for Local/Remote/Stop selector switches for switchgear (control circuitry that eventually uses a relay to keep the human's hand away from those nasty voltages)

  10. Kagato Says:

    "Hold on a second - Captain, exactly how many toggle flips in toto are involved in this procedure?"

  11. iworm Says:

    Once, when young, free, single and therefore wealthy (relative to today) I bought a Very Expensive Watch. Watch was (and still is) wonderful, but no one else seems to appreciate the box it came in. Beautiful bakelite. Gorgeous thing. Everyone thinks I'm quite mad. "It's a plastic box!" They cannot see the beauty. :-(

  12. Popup Says:

    When I graduated from University it was (and still is) the custom for men to wear white tie, i.e. tailcoat with all its trimmings. (This was in Sweden.)

    Most people rent it for the occasion, but I was fortunate enough to find one in my grandfather's cupboard that fit like a glove. It was probably made in the early 1930s, and still had its matching cufflinks in a pocket.

    The cufflinks were made out of mother-of-pearl and bakelite!

    On a related subject: I just read J.E Gordon's New Science of Strong Materials, where he claims that 'it's being said that the first commercial application of Bakelite was for the gear-knob of Rolls-Royce'. Do you know if that's true?

  13. Shadowex3 Says:

    Do you also have some mad scientist goggles and gloves? This thing would make a perfect light switch for a garage/work area. Put on the mad scientist outfit, slam the switch, lights come on...

  14. wongm Says:

    What happens when someone posts a link to this on a forum full of Victorian Railways enthusiasts?

    It's just a fancy light switch for a 90 odd year old carriage, it lets you have full or half lighting from the batteries. I will have to snap a shot of one in-situ next time I'm near one.

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