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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.


  1. Jack says:

    Edison used carbonized anthracite coal for his microphone and from what I’ve read, that material is still used for carbon microphones. See “Electronic Inventions and Discoveries: Electronics from its earliest beginnings to the present day,” Fourth Edition, page 91. You can read this page via Amazon’s “Search Inside” if you first search for the book and then use “search inside” for Edison.

    I’ve read somewhere – just can’t remember where – that Western Electric had a stockpile of some tons of specially selected Pennsylvania anthracite coal for microphones and that it was expected to last for 100 years or the like. (this comes from the time when carbon microphones were used in telephone instruments by the millions a year.)

    Around the same time Edison patented his carbon granule microphone (Patent no. US0222390), Emil Berliner patented a solid carbon microphone using a rod made from pencil lead, baked to remove volatile binders. Patent US0222652 and some of the steps required to convert pencil lead to a microphone form is described. Of course, this is pencil lead circa 1878, so the ingredients used today may well differ. Berliner’s microphone didn’t work as well as Edision’s design, though.

    In addition to an amplifier, a carbon microphone and headphone type transducer was used as an oscillator in early test equipment to generate low level AC voltage. These were called “hummers” and General Radio used them to generate 1000 Hz for it’s Model 650A RLC bridge, for example. (I have a 650A bridge in my collection, but it’s a slightly later version with AC power supply and vacuum tube oscillator.)

    Here’s what the General Radio Experimenter of April-May, 1933 said about the 650A’s 1000 Hz excitation source:

    “The internal batteries operate a microphone hummer
    for the production of the 1-kc current.”

    “The capacitance of this hummer to ground is small and has been allowed for in the bridge calibration.”


  2. Jack says:

    Found a useful reference from the Bell System Technical Journal, Vol 10, Jan 1931:

    “Condenser and Carbon Microphones-Their Construction and Use” by W.C. Jones

    It’s not the article I had in mind, but provides quite a bit of detail on high performance (at least as of 1931) carbon microphones.


  3. Tom R. says:

    I didn’t know what kind of carbon was used in telephone “transmitters” until reading Jack’s post above, but I am sure that carbon in many forms would have been available to those of the Steampunk era. I think I read in Henry Petroski’s book Pencil that graphite mining was a big industry in Victorian England, and I know that graphite can be used to make a carbon button mike.

    Back in the mid 1950’s my Father made a bi-directional microphone out of two telephone handsets for use in our church. It was placed so it could pick up the Pastor at the podium giving the sermon and also pick up the choir behind him at the same time. It did feed into a vacuum tube amplifier that was used to provide audio of the service to the room where there was a nursery for mothers with small babies and children. It would pick up a whisper from either the pastor or the choir.

    I played around with carbon button mikes a lot in the 1960’s and was always amazed at how sensitive they were. Especially for the voice frequencies,which is what they were designed for.

  4. Ashcat says:

    In the interest of science, and if you’ll send me postage costs after you receive the coal, I’d be happy to send you some anthracite–there are four tons in my basement. I have some bituminous too that I could send a sample of as well. Never made coke before but I’ll look into doing so–should be doable in my coal stove.

    Not sure whether you’re able to see my email address, but feel free to email me, Jeff. If not, reply here and we’ll figure some other way to exchange addresses.

  5. Lee Devlin says:

    Hi Jeff,

    I grew up in Wyoming Valley, PA, the ground zero of anthracite coal mining in this hemisphere (around Wilkes-Barre, PA) I actually have some of it here in Greeley. My largest sample is in the form of a paper weight, about 1.5 lbs. that is cut and polished to look like it is made of black diamond. I also have some small pieces of coal, just a few ounces. In measuring its conductivity, it’s on the order of a few hundred K Ohms, not exactly as conductive as a metal, more like a human body.

    Anthracite is considered the ‘purest’ coal, with the highest carbon content, about 60% greater than bituminous coal, or around 14,000 BTU/lb, vs. about 9000 BTU/lb for bituminous coal. I think you might do well to get some charcoal briquettes for your experiments. It’s just wood, heated in the absence oxygen that drives off the hydrogen, leaving mostly carbon and some ash. You can also make charcoal from sugar, and if you do, you’ll get something more like pure carbon.


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