The working principle of the microphone is:
when a person speaks into the microphone, sound waves make the diaphragm vibrate, the diaphragm suddenly loosens and squeezes the carbon particles, the resistance changes accordingly, and the sound vibration occurs in the circuit.
The changing current, the microphone,is equivalent to a variable resistor.
The working principle of the earpiece is:
when a current that changes in strength due to the vibration of the speaking sound is transmitted from the microphone,
the attraction of the electromagnet to the iron sheet also changes, causing the iron sheet to vibrate and produce the voice of speaking with the other party, with the same sound and the earpiece is equivalent to an electromagnet.