Bottle of ‘Battersea Carbon Granules’, No. 24 used to re-fill and repair early carbon microphones.

Carbon microphones use carbon granules to transform sound signal into an electrical current. Inside the microphone the granules are placed between two metal plates and behind a diaphragm. An electrical current is passed between the two plates. When sound waves hit the diaphragm it vibrates, placing pressure on the granules. The varying pressure compacting the granules changes the electrical resistance and creates a modulation of the current. This current can then be transferred as a signal down the wire.

These granules were manufactured by The Morgan Crucible Company who specialised in carbon components.