Coal That Doesn’t Conduct

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!