What is the primary use of granules of carbon in a telephone transmitter?

The primary use of granules of carbon in a telephone transmitter is to convert sound into electrical signals. This is a fundamental function of carbon microphones, where sound waves induce variations in pressure that compress or decompress the granules of carbon. These changes in pressure lead to variations in electrical resistance, effectively translating sound into an electrical signal that can be transmitted over distances.

The ability of the carbon granules to change resistance in response to sound makes them effective in early telephone transmitters, allowing clear communication by transforming sound vibrations into electrical currents that correspond to those sound waves. This process is crucial to ensure that what a person speaks into the transmitter is accurately conveyed through the telephone system, enabling two-way communication.