Science Sunday

I just measured the electrical resistance of ice.

Ordinary water-ice is a conductor, but not in the usual way. The charge-carriers in most electrically conductive substances are electrons and/or ions, but in water-ice they're protons - mobile hydrogen nuclei. Technically those protons do still count as ions, since they're a hydrogen atom without its electron, but proton conductivity is a distinctly different phenomenon from the usual kinds.

(I am indebted to the inimitable Bill Beaty - previously - for this information.)

So I was sitting here, and I thought, "let's see how conductive ice actually is".

First experiment: Plug multimeter probes into bench power supply.

(Just yesterday, I discovered that you can plug "shrouded" banana-plug multimeter probes into the usual sort of knobs-with-banana-sockets outputs from a power supply; just unscrew the knobs, and the probe-shroud fits neatly around the bare socket!)

Get ice cubes from fridge.

Wind power-supply up to maximum voltage (31V, for this eBay-cheapie supply). Stab probes into ice.

Current-display reading: Zero.

The scale on the bench supply bottoms out at 0.01 amps, though. Perhaps the resistance is just too high for 31 volts to be able to push 10 milliamps through it.

OK, let's try again.

Grab $10 yellow multimeter which for some reason I use much more often than my much more expensive Protek meter.

Set yellow multimeter to its highest resistance scale, which tops out at two megaohms. Stab probes into ice.

Reading: Off the scale, just like when the probes weren't touching the ice.

Suddenly remember that there is a reason why the Protek meter cost more. Its resistance mode tops out at 40 megaohms.

Set it to resistance mode, stab probes into ice.

Eureka! A reading!

With the probes separated by about an inch and only sticking into the ice a millimetre or two, I got a reading of about ten megaohms.

(I took care to avoid letting liquid water bridge the gap between the probes. The ice cubes were made from ordinary tapwater, and clean tapwater is a very lousy conductor - but once you're talking megaohms, all sorts of unlikely things are conductive enough to mess up a test like this.)

No wonder I didn't get a reading on the bench supply. 31 volts across ten million ohms gives a current of only 3.1 microamps. Even with the bench-supply probes really close together I was a few orders of magnitude short of the 10 milliamps that's the least the bench supply can display.

Perhaps it's not surprising that there are all those magic-water quacks. Water may seem to be a straightforward enough substance, but look just a little closer and it becomes as strange as electromagnetism. Hydrogen bonding, proton conductivity, a multitude of different kinds of ice, weird high-temperature, high-pressure behaviour... it goes on and on.

But I think there'd actually be just as much water woo-woo if water didn't do a single unexpected thing, not even expand when it froze. Crackpottery spontaneously generates all over the place, and bothers with scientific evidence only when some portion of that evidence can be used to support it.

(On the subject of ice, by the way, I highly recommend this book. It's full of gorgeous pictures of snowflakes, but it's not just another glossy coffee-table picture-book; it also has a lot of information about how ice forms and why it looks the way it does.)

14 Responses to “Science Sunday”

  1. ex-parrot Says:

    This was pretty neat - cheers :)

  2. NickL Says:

    That's actually really interesting.

    Also, I use my cheap, yellow, $15 meter all the time. I'd quite honestly be lost without it. I once accidentally dropped it into an open container of used motor oil. A little brake parts cleaner later, and it was good as new!

  3. Red October Says:

    Ah, cheap meters. Mine was free. It is from a company that does not show up on any web searches, for some reason, although I tested it against two other meters and it is very accurate. Possibly the best thing I ever got from my university's "Computer recycling program" (read: Here, students, faculty, alumni, have free shit since recycling costs money.)

  4. Tom Says:

    Your other option is to use the microamps scale on the protek and stick it in series with a power supply - 31 volts would give you a reading but something higher would be better. If you want to win a darwin award (i.e. dont do it), mains or rectified mains would work, and line-line from two phases, which you apparently have, would be even higher at about 400V.

  5. Steven Den Beste Says:

    Not to nitpick, but I'll nitpick: the charge carriers in liquid water are hydronium ions, (H3O+) and hydroxyl ions (HO-).

  6. Steven Den Beste Says:

    You mentioned that water is amazing and weird. 'Tis true, and by far the most amazing thing about it is that ice is less dense than liquid water. Few substances are like that, and if ice were more dense than water, life on earth would never have formed.

  7. xuth Says:

    You express concern about not conducting over surface water. Wouldn't the easiest way reduce this be to freeze two wires/probes into a block of ice such that the insulation is embedded in the ice. Then leave the ice in the freezer (with the wires sticking out) so that the heat transfer from the wire doesn't melt the ice and try to measure the resistance then.

  8. Kagato Says:

    "if ice were more dense than water, life on earth would never have formed."

    That's interesting. Can you elaborate on that, or point me towards further reading?

  9. j Says:

    Not to nitpick further, but liquid water isn't ice.

  10. shimavak Says:


    The basic idea is that, since water has its highest density (at ST) around 277 K, as you cool a large body of water the cold stuff will sink. As soon as the whole thing (baring thermal layers, of course) reaches 277K, the colder water will no longer sink. That means that it will freeze at the top first, and provide a not-insignificant insulation to the layers of liquid water beneath it. Thus, rather than the whole body of water freezing solid by the coldest stuffs continually sinking, you end up with just a crust freezing, and the rest staying a toasty 277K!

    Hope that explains it somewhat...

  11. Daniel Rutter Says:

    That means that it will freeze at the top first, and provide a not-insignificant insulation to the layers of liquid water beneath it

    ...which played a big role in the development of life on this planet. But we can't, of course, say for sure that the planet would be lifeless if water didn't behave in the way that it does. Perhaps land-based life would have developed by itself, perhaps water-based life would have developed when and where liquid water could still be found - deep oceans (climate permitting), underground water pockets, et cetera.

    I think it's fair to say, however, that if the only life on a planet exists in water pockets deep underground, that life's probably not going to evolve sentience.

  12. dr_w00t Says:

    It's a wonder the intelligent design crowd don't crow loudly about how strange water is. Maybe because science is the devil.

  13. Eats Big Dinners Says:

    Here's an interesting and detailed page on the anomalies of water ...

  14. Jonadab Says:

    > It’s a wonder the intelligent design crowd
    > don’t crow loudly about how strange water is.

    ICR does talk about that (or has in the past at any rate), but frankly it's only one of numerous factors that make Earth uniquely suited to life as we know it (carbon-chain-based blah blah blah). Earth's atmosphere is rare, for instance, in being adequate to support photosynthesis and respiration but transparent enough to allow enough light to penetrate to support photosynthesis, and to allow the stars to be seen (which is not so important for life but crucial for civilization). Our atmosphere also is rare in having enough diatomic oxygen (a powerful oxidizer) to support respiration but not quite enough to be really dangerous chemically (a few percentage points more would be quite hazardous). Once you start down that line of reasoning, the list becomes endless. The sun is just right. The moon is just right. Earth's gravity is just right. The ratios of the various elements is just right, which astronomers believe is not so true in most of the galaxy (though objectively we don't really know that for sure). The distance from the Sun to the Earth is absolutely perfect. The *amount* of liquid water is enough to create large oceans but not so much as to cover the whole surface. Et cetera, ad infinitum, ad nauseam.

    Of course you can interpret those data in various ways. You can say, "Well, duh, it was designed that way", or you can say, "It's perfect for us because we evolved and adapted under those conditions". Your conclusions depend not so much on the data as on the implicit assumptions you make going in. In other words, people believe what they want to believe.

Leave a Reply