Jeff Duntemann's Contrapositive Diary Rotating Header Image

December 10th, 2011:

I Want a Piece of Anthracite for Christmas…

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.