Jeff Duntemann's Contrapositive Diary

We got 10″ of new snow last night, and it’s still coming down. Given that it’s going to be a winter blunderland out on the streets today, we decided to stay inside and get a few things done closer to home. One of these things was to spend a little time measuring the resistance of coal.

Reader Joe Bryer was good enough to send me eight pounds of coal samples, in two Ziploc bags. One bag was high-quality Pennsylvania anthracite. The other was bituminous, its origins unspecified. To take the measurements, I made a jig out of a piece of scrap plastic. I drilled two .0785 holes a quarter inch apart. The distance was arbitrary. (The size of the holes is that of standard probe tips.) The idea was simply to make all measurements with the same distance between the probes.

I measured several spots on five pieces of bituminous, and in all cases the resistance was higher than my DVM’s top reading of 20 megohms. The anthracite was a completely different story. The resistance varied from place to place on any given sample, and varied widely even on a single sample. The highest readings I got were in the vicinity of 180K ohms. The lowest was 12K. I wiped the samples with a dry rag prior to testing them.

This is promising. The 1/4″ spacing between the probes was not quite arbitrary. The carbon button microphone element in an old-style telephone is about 1/4″ in size. I have several of those in the drawer and all of them tested at between 5K ohms and 10K, depending on how you orient the element, and whether you tap it with a screwdriver. It is, after all, just a little container of small carbon grains. So I think it would be worthwhile sifting some sand-sized granules of anthracite and trying them in a mic element lashup.

Joe Bryer pointed out something I hadn’t known: Bituminous and softer coals like lignite and peat are sedimentary rocks, whereas anthracite is metamorphic rock. Pressure and heat can break down some of the long carbon chains and ring hydrocarbons of which coal is made and free up the carbon, which can then, in a dense enough fragment, conduct electricity. Hydrocarbons, even very dense ones, don’t conduct electricity and many are very good insulators. Graphite, which conducts electricity very well, might be considered the highest grade of coal, or at least the variety most affected by heat and pressure underground. (It isn’t burned as fuel because it’s not common compared to coal and very difficult to ignite.)

The next thing to test is carbon from barbecue briquettes, which I will have to beg from friends since I no longer burn charcoal for grilling. If I can find some of the drafting pencil leads I used in drafting class umpty years ago, I’m going to grind them up and test them as well. (My friend Joe Flamini W4BXG says that he’s done that himself.) In the meantime, I have a place to start, and will now set my mind to building a microphone lashup to test conductive particles in. More as I manage it.

My friend George Ott surprised me with a lump of coal yesterday morning, after church over at St. Raphael’s. He even wrapped it and put a ribbon on it! I could barely stand the wait to get home and check its resistance. And (drum roll, please) the lump is…an insulator.

Or a damned reasonable facsimile. With my DVM set on its 20 megohm scale, the resistance didn’t read at all, even with the probes set pretty firmly in the coal about an eighth of an inch apart. I tested at eight or ten different points on the lump, since carbon is funny stuff generally and coal is not a uniform substance. The DVM reads a 10 megohm resistor right on the money, and failed to read a 22 megohm unit. I use that DVM a lot and certainly haven’t seen anything wrong with it. But no matter how close I set the probes, I got no reading at all.

I put the lump against my belt sander and ground a flat face, which was interesting in terms of the tarry smell that came off the lump. No difference. I whacked a corner off the lump with a coal chisel. No difference, even on the fresh and very shiny fracture face. With the clean and very sharp points of the probes no more than a sixteenth of an inch apart, the resistance was higher than 20 megs. Wow.

I’m not enough of a rockhound to know what sort of coal it is, and George didn’t know either. (He got it from a blacksmith who fires his forge with coal.) It looks like anthracite in that it’s quite shiny, but beyond that I just don’t know.

Another of my readers is sending me a sample of both anthracite and bituminous, and that may help. Some of the online sources I’ve read describe coal as a semiconductor, but I think by that they only mean a substance with significant resistance. I’ve seen nothing to indicate that some types of coal would be insulators across very small fractions of an inch.

However, among other projects I hope to get to this winter is a crystal detector lashup, steampunk style, on an oak base with a copper pipe cap to hold a mineral sample for testing as a cat-whisker detector. One doesn’t think of coal as a candidate, but it certainly won’t take much work to do the science!

Yes, I want a lump of coal for Christmas. Two actually: Santa, if you’re listening, see if you can get me four or five ounces of a good glossy anthracite, and a similar quantity of mid-grade bituminous. I’m not sure where else I’m likely to find it. (Ok, ok, eBay. Last resort…)

I haven’t been bad. I’ve been curious.

A question occurred to me the other day while I was hauling boxes around the lower level: Does coal conduct electricity? And if so, how well? I know that carbon does, but of course you have to specify which kind of carbon. Diamonds do not conduct electricity. Carol’s engagement ring is big enough for me to have put an ohmmeter across its large facet thirty-odd years ago. The carbon rods running down the centers of conventional carbon-zinc batteries conduct very well. Mechanical-pencil lead conducts electricity. Resistors, in fact, were basically painted lengths of pencil lead until relatively recently.

Coal, now. Hmmm. I would run downstairs and do the science right this minute, but I’m not sure I’ve held a piece of coal in my hand for forty years. Uncle Joe Labuda burned anthracite in a coal stove to heat his flat down Back of the Yards around 1960. I was fascinated by the lumps of coal in the bin behind his stove for their luster and even more by their smell, which was a less acrid form of the coal smoke that hovered over the neighborhood all winter, at least until people started installing natural gas space heaters.

Online research suggests that the resistance measured across the thickness of a one-centimeter cube of anthracite runs from the mid-hundreds to low thousands of ohms. That suggests that sand-grain sized particles of coal could be used to create a carbon button microphone. My friend Art Krumrey actually did this circa 1963, by beating on a carbon rod yanked out of a dead flashlight battery, and loading the grains between a soda bottle cap and half of one of the thin steel puck-shaped containers that large rolls of Scotch tape used to come in. He basically duplicated the circuitry of a primordial telephone connection, and we sent our voices over fifty feet of wire without any active devices at all, just a few dry cells in series with the cobbled-up carbon mic and a pair of dynamic headphones. It was boggling how loud the audio was, so loud that it overloaded the headset unless we spoke in practically a whisper.

This was how telephone systems worked before amplifier tubes were invented: The voice audio signal coming out of the carbon mic was already high-level, being a variable resistance in series with a high-current power source. In fact, electromechanical repeater amplifiers were created by mechanically coupling a dynamic earpiece to a carbon button mic. With decent batteries to drive the system, repeater chains like that could carry voice signals hundreds of miles without a vacuum tube in sight.

What I really want to know is whether a steampunk-era garage inventor could have created a usable carbon button mic using granules of coal, of if purer carbon would be required. If I can find a lump of coke that might work better, and turning coal to coke is not exactly alchemy. (I’m not sure I want to do it myself.) If the resistance of a lump of good anthracite were low enough, it could also function as the cathode of a carbon-zinc primary battery, which would be interesting all by itself. Water-cooled carbon mics the size of pie pans were used in series with high-speed alternators to generate voice-modulated RF before the advent of RF power tubes. It’s a delicious steampunkish concept, full of sparks and ozone and odd things turning too quickly for their own good.

Of course, I have a Drumlins World story concept that involves simple electromechanical wireless voice transmission systems. The sinister Bitspace Institute has a very secret radio communications network, and when a pair of spindly teenage boys independently invent spark radio, well, interesting things happen–especially when you throw a few drumlins into the mix.

Still taking notes, but even a few ounces of good coal could make for some interesting experiments, just as my steampunk Geiger counter did last year. Once the lower level is done I hope to lash something up to measure the effectiveness of different kinds of carbon granules in microphone service. Whether the story itself gets written or not, I expect to learn something, and that’s good enough for me.