"Iain Churches" wrote

Did you find out why the type 21 failed?


Yes, whilst the 13 had used solid carbon electrodes, the 21 used
"carbonised" steel electrodes. Due to surface contamination on the metal
prior to carbonisation there was not always a satisfactory bond at the
molecular level between the steel and the carbon, this allowed the surface
of the steel to corrode under the carbon layer. The solution was to improve
the cleaning of the surface of the steel prior to carbonisation.

Interestingly there was, even in the 1960s by which time when carbon
microphones had been in common use for some 80 years, no really satisfactory
theory of how they worked. The common theory at the time was that the
contact area between adjacent granules increased with increasing pressure,
but this could not explain why the *average* resistance increased with
increasing sound level. The theory that the group I was working in arrived
at was that the granules bounced off each other whilst activated by sound,
so that each point of contact became a sort of pulse-width modulator.

David.