Carbon microphones

The Reisz carbon microphone - How it works:

a container with carbon granules, closed of by a rubber membrane, with two electrodes inserted, is powered by an electrical current. When sound reaches the microphone, the carbon granules are pressed together. The changing current is further amplified and results in a reasonable sound quality.

Carbon mics were the first professional microphones and were used in early Radio transmissions, they were also known as "transmitters".

Originally the first microphones that were used were converted telephones, which were also carbon based.

Their pick-up pattern is Omni.