I recently came across a blog post by a gentleman in Colorado expressing his wish to locate some anthracite coal for Christmas. Not to put in anyone's stocking, but rather to try to build a carbon microphone, another of Edison's inventions. He had no leads on acquiring anthracite.
For nearly 100 years, until the 1970's-80's, carbon mics were used in telephones everywhere. They consist of two thin metallic/conductive electrode plates (for example, made of aluminum) with fine particles of carbon, usually obtained from acid-treated anthracite, in between them. These particles have some electrical conductive ability, and sound (voice) causes vibrations that modify the distribution of carbon particles within the space between the electrodes, which modifies the amount of current passing from one plate to the other. Typically the carbon-containing space is sealed at the edges, producing a "button" with powdered anthracite inside--hence the common name 'carbon button microphone'. These microphones have some inherent signal amplifying properties, so they can be used with much lower voltages than modern electronic microphones. For this reason they are still used in some military and industrial applications where this property is desirable.
This is yet another, "new" application of coal that I'd never known of before. With interest in supporting such explorations (tinkering) on anyone's part, I contacted him and offered, in exchange for my postage costs, to send him 5 lbs of anthracite and, as he requested and since I have a little of it lying around, about 3 lbs of bit. I sent it yesterday and it's on its way. He also expressed curiosity in his original post whether coke, having very high carbon content (88-92%), would be an even better source of carbon particles than raw anthracite. My guess, just based on carbon content, is that it should be about equally desirable to anthracite. But, since the particles have to be quite small, perhaps coke would be preferable in grinding to fine enough particles.
I didn't have any coke to send along, but got to wondering how I might make some. From what I've read, it's made from heating bit coal to about 2000* F in an oxygen-deficient environment. One place that's mostly oxygen-deficient is inside an anthracite coal stove not long after loading--much or most of the oxygen becoming bound to carbon during combustion to produce carbon dioxide and carbon monoxide. Conveniently, that same area also happens to be pretty hot (Greg L reported here that his highest reading on a probe thermometer in his coalbed was about 2200* F). So, a small (steel?) vented container of bit coal, resting on the fire itself, might do the trick. I'm thinking that shortly after loading fresh coal, leaving an area of open fire on which to rest the container, might be ideal. This is the time when there's probably the least amount of oxygen inside the firebox, and the best time to open up the draft very wide (allowing very high temperature of the coal immediately below the container). I'm not sure how long the bit coal would have to "cook" before it became coke.
Any thoughts on these methods? Has anyone ever tried to make coke?
(Richard or other Moderators: if this post belongs on the Bituminous forum, feel free to move it)
If interested in carbon mikes: http://en.wikipedia.org/wiki/Carbon_microphone
http://books.google.com/books?id=fcOAKt ... ne&f=false