4. On the Cause of the Sounds produced in the Microphone Receiver

Extract

In the microphone, or loose contact receiver, we have two or more small blocks of carbon resting lightly against each other, through which the interrupted current from the transmitter is passed. At the points of contact, a strong heating and cooling effect must take place every time the current is interrupted or otherwise varied. This heating and cooling will cause corresponding expansions and contractions of the air or other medium surrounding the points. These produce undulations in the medium, which, striking against the sides of the containing vessel, set them also into a state of vibration, and so reproduce the sounds of the transmitter.