Which material is found in an ordinary telephone transmitter?

Granules of carbon are found in an ordinary telephone transmitter due to their ability to convert sound waves into electrical signals and vice versa. In traditional carbon-button transmitters, sound waves from a speaker cause a diaphragm to vibrate. These vibrations compress and decompress the carbon granules, which alters the electrical resistance in the circuit. This change in resistance is translated into electrical signals that can be transmitted over telephone lines.

This specific functionality of carbon granules gives them a significant advantage in performance for sound transmission, enabling clarity and quality in communication. The other materials listed do not possess the same properties that facilitate this conversion process in the context of a telephone transmitter.