The telephone (from the Greek words tele = far and phone = voice) is a telecommunications device that is used to
transmit and receive sound (most commonly speech), usually two people conversing but occasionally three or more. It is one of the most
common household appliances in the world today. Most telephones operate through transmission of electric signals over a complex telephone
network which allows almost any phone user to communicate with almost anyone.
A traditional landline telephone system, also known as "plain old telephone service" (POTS), commonly handles both signaling and audio information
on the same twisted pair of insulated wires: the telephone line. Although originally designed for voice communication, the system has been adapted
for data communication such as Telex, Fax and Internet communication. The signaling equipment consists of a bell, beeper, light or other device to
alert the user to incoming calls, and number buttons or a rotary dial to enter a telephone number for outgoing calls. A twisted pair line is
preferred as it is more effective at rejecting electromagnetic interference (EMI) and crosstalk than an untwisted pair.
A calling party wishing to speak to another party will pick up the telephone's handset, thus operating a button switch or "switchhook", which puts
the telephone into an active state or "off hook" by connecting the transmitter (microphone), receiver (speaker) and related audio components to the
line. This circuitry has a low resistance (less than 300 Ohms) which causes DC current (48 volts, nominal) from the telephone exchange to flow
through the line. The exchange detects this DC current, attaches a digit receiver circuit to the line, and sends a dial tone to indicate readiness.
On a modern telephone, the calling party then presses the number buttons in a sequence corresponding to the telephone number of the called party.
The buttons are connected to a tone generator that produces DTMF tones which are sent to the exchange. A rotary dial telephone employs pulse dialing,
sending electrical pulses corresponding to the telephone number to the exchange. (Most exchanges are still equipped to handle pulse dialing.)
Provided the called party's line is not already active or "busy", the exchange sends an intermittent ringing signal (generally over 100 volts AC)
to alert the called party to an incoming call. If the called party's line is active, the exchange sends a busy signal to the calling party. However,
if the called party's line is active but has call waiting installed, the exchange sends an intermittent audible tone to the called party to indicate
an incoming call.
When a landline phone is inactive or "on hook", its alerting device is connected across the line through a capacitor, which prevents DC current
from flowing through the line. The circuitry at the telephone exchange detects the absence of DC current flow and thus that the phone is on hook
with only the alerting device electrically connected to the line. When a party initiates a call to this line, the ringing signal transmitted by
the telephone exchange activates the alerting device on the line. When the called party picks up the handset, the switchhook disconnects the
alerting device and connects the audio circuitry to the line. The resulting low resistance now causes DC current to flow through this line,
confirming that the called phone is now active. Both phones being active and connected through the exchange, the parties may now converse as
long as both phones remain off hook. When a party "hangs up", placing the handset back on the cradle or hook, DC current ceases to flow in
that line, signaling the exchange to disconnect the call.
Calls to parties beyond the local exchange are carried over "trunk" lines which establish connections between exchanges. In modern telephone
networks, fiber-optic cable and digital technology are often employed in such connections. Satellite technology may be used for communication
over very long distances.
In most telephones, the transmitter and receiver (microphone and speaker) are located in the handset, although in a speakerphone these components
may be located in the base or in a separate enclosure. Powered by the line, the transmitter produces an electric current whose voltage varies in
response to the sound waves arriving at its diaphragm. The resulting current is transmitted along the telephone line to the local exchange then
on to the other phone (via the local exchange or a larger network), where it passes through the coil of the receiver. The varying voltage in the
coil produces a corresponding movement of the receiver's diaphragm, reproducing the sound waves present at the transmitter.
A Lineman's handset is a telephone designed for testing the telephone network, and may be attached directly to aerial lines and other
infrastructure components.
History of the Telephone
Source References:
- Coe, Lewis (1995), The Telephone and Its Several Inventors: A History, McFarland, North Carolina, 1995. ISBN 0-7864-0138-9
- Evenson, A. Edward (2000), The Telephone Patent Conspiracy of 1876: The Elisha Gray - Alexander Bell Controversy, McFarland, North Carolina, 2000. ISBN 0-7864-0883-9
- Baker, Burton H. (2000), The Gray Matter: The Forgotten Story of the Telephone, Telepress, St. Joseph, MI, 2000. ISBN 0-615-11329-X
- Huurdeman, Anton A. (2003), The Worldwide History of Telecommunications, IEEE Press and J. Wiley & Sons, 2003. ISBN 0-471-20505-2
- Josephson, Matthew (1992), Edison: A Biography, Wiley, 1992. ISBN 0-471-54806-5
- Bruce, Robert V. (1990), Alexander Graham Bell and the Conquest of Solitude, Cornell University Press, Ithaca, 1990.
- Wikipedia.org, the free encyclopedia
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