"David Looser" wrote in message
...
"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.

So does that mean that simpler was better, i.e more reliable?

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,

Carbon mics were not used in recording, but they were
described briefly in text books. The explanation you
write above is the one I remember reading.

Funny how I had almost forgotten about that old mic, and
all the fun we had with it (simple pleasures back in those
days:-) On Thursday evening my Dad and I used to sit
at the kitchen table and "make something, sometimes
it was a model ship, or plane, sometimes an amp or a
simple one valve radio. This mic was used with a one-valve
mic that had an anode cap - (pretty dangerous thing for
a pre-teen to be using), and a Rola speaker salvaged
from a "radio-gram" The amp was built in an Oxo tin chassis!
Later, the boy next door and I rigged a wire from the ground
terminal to a pole in the garden and fooled the other boys
into thinking it was a transmitter:-)


Shortly after that they packed me off to boarding school:-)
I joined the radio club and they taught me how to charge
up electrolytics. We we used to roll them under the beds
for the dorm-maids to find:-)

Iain