[Boatanchors] Coke as a detector - was: Foxhole Radios

[Barry L. Ornitz]

There has been considerable discussion on the use of coke as a 
detector in simple crystal sets.  It is cetainly possible that 
an anthracite coal might have some impurities that would act 
as a detector, but the original article specifically said 
coke.  In coal, the sulfur occurs in three forms: pyrites, 
organic compounds, and sulfates.  Metal pyrites are 
semiconductors and they have been extensively used as 
detectors so it may certainly be possible to find "sweet 
spots" in a lump of coal too.

Coke, however, is the solid carbon and ash containing residue 
left after the high temperature distillation drives all the 
moisture and volatile organic matter out.  The coal is heated 
in the absence of air, and most of the complex organic 
compounds in the coal are broken down.  Various combustible 
gases, oils and tars are evolved during the heating.  What is 
left is the coke.

Different coals (anthracite, bituminous, lignite, etc.) 
contain differing amounts of moisture, volatile matter, fixed 
carbon, and ash.  Ash is what is left after all of the carbon 
is combusted, and it is composed of a mixture of all common 
elements.  The ash content of some coals is quite high; for 
example coal from the Pennsylvania Mammoth seam [anthracite] 
has about 7% ash and 3% volatiles, while closer to my home the 
Kentucky Elkhorn seam [bituminous] has about 3% ash and 35% 
volatiles.

One of the more common ingredients of the ash is silica or 
silicon dioxide - ordinary beach sand in chemical content.  
When heated at high temperatures with the carbon, some of the 
silica is reduced producing carbon dioxide and carbon monoxide 
and silicon carbide.  This is definitely a semiconductor.  It 
is currently used in some light emitting diodes and has been 
proposed for semiconductors that must operate at temperatures 
well above where silicon can be used.  So it is very likely 
that a piece of coke would have small crystals of silicon 
carbide which will act as detectors.

Lots of natural semiconductors are found in nature and can be 
used as detectors.  Galena (lead sulfide) and iron pyrite are 
common ones.  But two dissimilar metals with some galvanic 
corrosion between them can also act as rectifying junctions.  
I am sure most of us are aware of the problem with loose, 
rusty bolts in guy wires and such rectifying nearby strong RF 
fields and producing harmonics and mixing.

I would like to conclude by saying that silicon and germanium 
semiconductors were in use in radios long before vacuum tubes 
were invented.  Even semiconductor oscillators were described 
in the literature and documented - well before vacuum tubes.  
So please think twice before cursing semiconductors as new-
fangled technology!  :-)

        73,  Dr. Barry L. Ornitz     WA4VZQ     ornitz@tricon.net