Telephones
How they work: a history

       by Andy White

Background

The first words spoken over the telephone were on March 10th 1876. They were those of its inventor, Alexandra Graham Bell.

The following is taken from the entry made in his notebook that day:

Graham Bell in 1876

Volume 1. March 10, 1876

Fig. I

The improved instrument shown in Fig. I was constructed this morning and tried this evening...

…Mr. Watson was stationed in one room with the Receiving Instrument [(S)]. He pressed one ear closely against S and closed his other ear with his hand. The Transmitting Instrument was placed in another room and the doors of both rooms were closed.

I then shouted into M the following sentence: "Mr. Watson - Come here - I want to see you". To my delight he came and declared that he had heard and understood what I said.

[Bell, Unpublished Notebook, Vol. I, p40-41]

 

The basic operation of the telephone has hardly changed since that time. In fact the old dial phones (fig 2), are hardly any different in operation to the improved telephone designs made in 1877.

Figure 1.. A 700 series telephone
(The 700 series telephones were introduced in 1956 to replace the old Bakelite telephones. They ceased being marketed in 1986)

How a telephone works

A telephone set consists of six fundamental components:

• Transmitter (the telephone mouthpiece, or microphone)
• Receiver (the earpiece, or 'speaker')
• Dial or Keypad
• Switch hooks. (A switch)
• Bell or 'tone caller'

Figure 2. Inside a telephone: A simplified view.

With the handset is down, only the bell is connected across the telephone line. When the handset is picked up the receiver, transmitter and dial are also connected.

When someone calls you, the local telephone exchange sends out an electrical 'ringing' current. This current travels along the telephone line and into your telephone. The 'bell' will now ring.

Picking your handset up causes a switch to come on (the switch hook closes). This completes a circuit that the exchange can detect. If your phone was ringing, the exchange connects you to the caller; otherwise it connects you to a 'dial tone'.

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