How Telephones Work

Telephone technology may have changed over the decades, but the basic principles have not. Here's how a basic telephone system works.

In order to understand how telephones work, it may pay to return to childhood for a moment. Our most basic phone system consisted of two paper cups and a generous length of string. The string would be tied through the cups' bottoms and held as tightly as possible. One child would shout into his cup as the other held the other cup to his ear. The sound quality may have been questionable, but there was no doubt that communication had indeed taken place over a distance. As long as the tension was maintained and both parties were aware of the 'call', this primitive phone system worked well.

Modern telephone technology, broadly speaking, is only a better version of the original cups and string. Alexander Graham Bell and others were already aware of an earlier invention called the telegraph. Electricity could travel a substantial distance over metal wires, especially if it were periodically boosted by batteries. Since electrical current could be turned on and off by switches, telegraph operators sent text messages in a series of short and long pulses that trained human receivers could reinterpret. The system was very limited, relative expensive to maintain and only a handful of trained operators could reliably decode messages. Telegraphs installed in private homes would be impractical and too costly.

Bell and those who followed realized that if electricity could be used to send a simple on-off code, it might also be able to send more complicated information. If we re-examine the cups and string for a moment, we can understand the dilemma. The human speaker generated soundwaves, most of which were directed at the bottom of his paper cup. This caused the thin paper to vibrate in a specific range. The taut string also vibrated because of the attachment. The string became a carrier of the soundwaves, but not in the same form as the original speaker. When the string's vibrations reached the other cup, the cup's thin paper bottom also began to vibrate in approximately the same range as the original speaker. These new vibrations disturbed the air in the cup and eventually reached the listener's ear. His brain processed those signals into recognizable speech.



In a standard phone, the cups are replaced with an electronic speaker in the earpiece and a electronic transmitter in the mouthpiece. The string has now become a set of wires maintained by the phone company. In some areas, the wires may be replaced by microwave towers or fiber optic lines, but the principle is still the same. Another modification is the addition of a ringer which alerts a phone user whenever an incoming call is received.

So here's how a typical telephone call works. The caller picks up the handset containing a speaker and transmitter. Once the phone is lifted from the cradle or switched on, a circuit is completed at the local phone company's offices. If a line is available, the phone company's computer sends out a 'dial tone' which basically tells the user that the system is ready to receive further instructions. Most phones have tone generators which correspond to numbers on a keypad. The user pushes a series of numbers on the keypad, which in turns sends out the proper tones to the computerized switcher at the phone company.

Once the computer recognizes the number and logs the details of the call, a connection is made between the caller's line and the line leading to the intended receiver. A 48 volt battery system maintained by the phone company sends power to the receiver's ringer system. Once the receiver picks up his own handset, the voltage is reduced considerably. The receiver greets the caller and the connection is made. This connection will continue as long as the switching equipment senses that both parties are on the line. Hanging up the phone should cause the call to be terminated, but there is often a 10 second grace period before a call is actually disconnected.

The caller begins to speak into a miniature microphone located in the mouthpiece of the handset or base of the phone. His voice is converted to electrical pulses, which travel through the wire to the receiver's speaker located in the earpiece. The sound is rarely equivalent to the original speaker's voice, because the electrical pulses are received in a very limited range of sound. The result is a recognizable, if slightly 'squashed', version of the speaker's message. The conversation can then be reversed, as the receiver initiates a conversation through his own mouthpiece and the original caller picks up the signals in his earpiece.

The basic telephone service begins and ends with one party speaking to another party. Over the years, phone companies and private inventors discovered that customers had a need for other services. Phones which could handle more than one incoming call proved useful for businesses, while private customers wanted to be able to have three-way conversations or be notified of other incoming calls. Certain modifications benefited the phone companies themselves, such as switching from a rotary dial system to a more efficient touch-tone system. Wireless systems have replaced cords in many telephones, and the capacity of an individual line has been increased to accommodate computer modems and electronic money systems. But the basic system underlying all this technology is primarily the same as it was during Alexander Graham Bell's day.

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