Using this microphone requires the wearing of a dinner jacket and a black bow tie! (or a Fascist military uniform!) Designed by the young George Neumann, working for Eugen Reisz, before he saw sense, and went on to make condensers! It is carved out of a solid octagonal block of marble, and filled with carbon granules (coal dust) behind a mica diaphragm. (Short sound clip.)
View inside my Marconi-Riesz with the front cover and wind screen removed. The thin mica diaphragm is glued on to the marble and the chamber containing the carbon granules is clearly visible. The screw holes in the marble have neat little fibre rawl plugs to hold the screws tight.
A great look for a mad Italian dictator!
Eh!… Benito I’m likin’ da mics!
Below is a simple circuit diagram for a pre-amp to power carbon microphones
Also an interesting technical article.