How a telephone handset works.

Emitter

The transmitter is located in the microphone position of the telephone handset, and it converts the speaker's voice into electrical pulses (specifically, fluctuating DC currents) so that it can be transmitted to the receiver. The receiver is located in the headphone position of the telephone handset, and it does the opposite job. It takes in electrical pulses and converts them into sound that the listener can understand.

The earliest telephones used springs, thin vibrating plates, or liquid filled carbon boxes to convert electrical pulses into sound. However, the most common type of telephone transmitter in the 20th century was the carbon pellet bag invented by Thomas Edison. Because these transmitters are economical, they are still used in some telephones.

For carbon-filled emitters. The DC voltage applied to the carbon particles compresses them, changing the amount of electricity that can pass through them so that it corresponds to the speed of the signal. In early telephone technology, this fluctuating cable was sent to the receiver through a central office, where an operator completed the circuitry. It starts as an analog signal and remains an analog signal on its way to its destination [1].

The fluctuating DC current is converted to a digital signal by the local office switch that first receives the current. After the switch of the same telephone company, the signal is converted to analog form and sent to the transmitter. For the most part, telephones no longer use compressed carbon particles in their transmitters. Instead, they use tiny electronic microphones. However, the signals from telephone microphones are still analog and must be converted to a digital format so that computerized devices can interpret them.

Receiver

In the last century, telephone receivers have changed less than transmitters. Early receivers used a vibrating diaphragm, similar to a stereo speaker, but much smaller. The incoming DC current causes the electromagnetic coil next to the diaphragm to emit a wave. When the diaphragm vibrates in response to these waves, it produces a sound like speech. Many telephone receivers still use this technology. However, some receivers have been replaced by smaller, lighter electronic components.