The telephone dates from 1876, when Alexander Graham Bell patented the first practical instrument. Bell's telephone consisted of an electromagnet and a metal diaphragm. Two such instruments were required, one for transmitting and another for receiving. These were connected by wire to an exchange. Shortly afterwards, Bell's electromagnetic microphone was replaced by the far superior carbon microphone patented by Thomas Alva Edison.
The microphone converts the sound waves of the sender's voice (variations in air pressure) into an electrical signal (variations in current) which can be transmitted along a pair of wires to the receiver, where they are converted back into sound waves by the earpiece. Although the basic components remain the same - transmitter/microphone (mouthpiece) and receiver (earpiece) - the telephone's appearance has changed considerably since Bell's time.
The next major development was in the exchange that was necessary to connect widely dispersed telephone users to each other. The automation of this process was initiated in 1889 by Almon B Strowger, a Kansas City undertaker who was facing bankruptcy. He believed that an operator in the local telephone exchange was diverting his business enquiries to her relative, a rival undertaker. This called for drastic action to eliminate the human element! After much experimenting, with circular shirt-collar boxes and matchsticks Strowger came up with an ingenious design of a rotating switch with any number of contacts - one for each line. The switch was moved magnetically. Engaged lines contained certain voltages which powered the switch to move on until it reached a free line. This formed the basis of all electromagnetic exchanges used throughout the world for over a century.
Shortly before the turn of the 20th century, the candlestick telephone was introduced. This used a magneto system, which meant that the phone was connected to a large wooden box called a "subset" containing a battery, bell, and crank. The all-in-one hand combination set, combining microphone and earpiece in one handset, appeared in 1929. It looked modern and was more comfortable to use. Improvements over the years led to various lightweight moulded plastic designs produced in many colours.
The use of short-range radio links for telephones has created the mobile phone networks which are now so familiar. While these are sophisticated ultra modern systems, we should remember that they use the theory of radio communication based on the pioneering research of James Clerk Maxwell and have the same basic elements as the telephone of Alexander Graham Bell.