telephone
 Encyclopædia
Britannica Article |
an instrument that is designed for the
simultaneous transmission and reception of the human voice.
Inexpensive, simple to operate, and offering its user a personal
type of communication that cannot be obtained through the written
word, the telephone has become the most widely used
telecommunications device. Hundreds of millions of telephone sets
are in use throughout the world. Each business day almost two
billion telephone transmissions take place in the United States
alone.
A brief description of the telephone
instrument follows. For full treatment of the development of the
telephone instrument and the telephone system, see Telecommunications
Systems: Telephone.
For international statistical data on
telephones, see Britannica Book Of The
Year.
The word telephone, from the Greek roots
tele, “far,” and phone, “sound,” was
applied as early as the late 17th century to the string telephone
familiar to children and was later used to refer to the megaphone
and the speaking tube; but in modern usage it refers solely to
electrical devices derived from the inventions of Alexander
Graham Bell and others. The U.S. patent granted to Bell in March
1876 (No. 174,465) for the development of a device to transmit
speech sounds over electric wires is often said to be the most
valuable ever issued. The general concepts involved in the invention
of the telephone—of speech sounds as a complex of vibrations in air
that is transferrable to solid bodies and of the convertibility of
those vibrations to electrical impulses in conducting metals—had by
then been understood for decades. Bell was but one of a number of
workers racing to pull them together into a practical instrument for
the transmission of speech.
Within 20 years of the Bell patent, the
telephone instrument, as modified by Thomas Watson, Emil Berliner,
Thomas Edison, and others, acquired a form that has not changed
fundamentally in a century. Since the invention of the transistor in
1947, metal wiring and other heavy hardware have been replaced by
lightweight and compact microcircuitry. Advances in electronics have
improved the performance of the basic design, and they also have
allowed the introduction of a number of “smart” features such as
automatic redialing, call-number identification, and
analog-to-digital conversion for transmission over digital circuits.
Such advances supplement, but do not replace, the basic telephone
design. As it has since the early years of telephone communication,
the telephone instrument comprises the following functional
components: a power source, a switch hook, a dialer, a ringer, a
transmitter, a receiver, and an anti-sidetone circuit. These
components are described in turn below.
Power source. In the first
experimental telephones, the electric
current that powered the circuit was
generated at the transmitter, by means of an electromagnet activated
by the speaker's voice. Such a system could not generate enough
voltage to produce audible speech in distant receivers, so every
transmitter since Bell's patented design has operated by producing
variations in a direct current supplied by an independent power
source. The first sources were batteries
located in the telephone instruments themselves, but since the 1890s
current has been generated at the local switching office. The current is supplied
through a two-wire circuit called the local loop. The standard voltage in the Bell
system is 48 volts.
Cordless telephones represent a return to
individual power sources in that their low-wattage radio
transmitters are powered by a small (e.g., 3.6-volt) battery
located in the portable handset. When the telephone is not in use,
the battery is recharged through contacts with the base unit. The
base unit is powered by a transformer connection to a standard
electric outlet.
Switch hook. The switch hook connects the
telephone instrument to the direct current supplied through the
local loop. In early telephones the receiver was hung on a hook that
operated the switch by opening and closing a metal contact. This
system is still common, though the hook has been replaced by a
cradle to hold the combined handset (enclosing both receiver and
transmitter). In some modern electronic instruments, the mechanical
operation of metal contacts has been replaced by a system of
transistor relays.
When the telephone is “on hook,” contact with
the local loop is broken. When it is “off hook” (i.e., the
handset is lifted from the cradle), contact is restored, and current
flows through the loop. The switching office signals restoration of
contact by transmitting a low-frequency “dial tone” (actually two simultaneous tones of
350 and 440 hertz).
Dialer. The dialer is used to enter the number of the party
that the user wishes to call. Signals generated by the dialer
activate switches in the local office, which establish a
transmission path to the called party. Dialers are of the rotary and push-button types.
The traditional rotary dialer, invented in
the 1890s, is rotated against the tension of a spring and then
released, whereupon it returns to its position at a rate controlled
by a mechanical governor. The return rotation causes a switch to
open and close, producing interruptions, or pulses, in the flow of
direct current to the switching office. Each pulse lasts
approximately one-tenth of a second; the number of pulses signals
the number being dialed.
In push-button dialing, introduced in the 1960s,
the pressing of each button generates a “dual-tone” signal that is specific to the
number being entered. Each dual tone is composed of a low frequency
(697, 770, 852, or 941 hertz) and a high frequency (1,209, 1,336, or
1,477 hertz), which are sensed and decoded at the switching office.
Unlike the low-frequency rotary pulses, dual tones can travel
through the telephone system, so that push-button telephones can be
used to activate automated functions at the other end of the
line.
In both rotary and push-button systems, a
capacitor and resistor prevent dialing signals from passing into the
ringer circuit.
Ringer. The ringer alerts the user to
an incoming call by emitting an audible tone or ring. Ringers are of
two types, mechanical or electronic. Both types are activated by a
20-hertz, 75-volt alternating current generated by the switching
office. In the Bell system, the ringer is activated in two-second
pulses, each pulse separated by a pause of four seconds.
The traditional mechanical ringer was introduced with the early
Bell telephones. It consists of two closely spaced bells, a metal
clapper, and a magnet. Passage of alternating current through a coil
of wire produces alternations in the magnetic attraction exerted on
the clapper, so that it vibrates rapidly and loudly against the
bells. Volume can be muted by a switch that places a mechanical
damper against the bells.
In modern electronic ringers, introduced in the 1980s,
the ringer current is passed through an oscillator,
which adjusts the current to the precise frequency required to
activate a piezoelectric transducer (a device made of a
crystalline material that vibrates in response to an electric
current). The transducer may be coupled to a small loudspeaker,
which can be adjusted for volume.
The ringer circuit remains connected to the
local loop even when the telephone is on hook. A larger voltage is
necessary to activate the ringer because the ringer circuit is made
with a high electrical impedance in order to avoid draining power
from the transmitter-receiver circuit when the telephone is in use.
A capacitor prevents direct current from passing through the ringer
once the handset has been lifted off the switch hook.
Transmitter. The transmitter is essentially
a tiny microphone located in the mouthpiece of the telephone's
handset. It converts the vibrations of the speaker's voice into
variations in the direct current flowing through the set from the
power source.
In traditional carbon transmitters, developed in the 1880s, a
thin layer of carbon granules separates a fixed electrode from a diaphragm-activated electrode. Electric current
flows through the carbon against a certain resistance. The
diaphragm, vibrating in response to the speaker's voice, forces the
movable electrode to exert a fluctuating pressure on the carbon
layer. Fluctuations in the carbon layer create fluctuations in its
electrical resistance, which in turn produce fluctuations in the
electric current.
In modern electret transmitters, developed in the 1970s,
the carbon layer is replaced by a thin plastic sheet that has been
given a conductive metallic coating on one side. The plastic
separates that coating from another metal electrode and maintains an
electric field between them. Vibrations caused by speech produce
fluctuations in the electric field, which in turn produce small
variations in voltage. The voltages are amplified for transmission
over the telephone line.
Receiver. The receiver is located in the
earpiece of the telephone's handset. Operating on electromagnetic
principles that were known in Bell's day, it converts fluctuating
electric current into sound waves
that reproduce human speech. Fundamentally, it consists of two
parts: a permanent magnet having pole pieces wound with coils of
insulated fine wire, and a diaphragm driven by magnetic material
that is supported near the pole pieces. Speech currents passing
through the coils vary the attraction of the permanent magnet for
the diaphragm, causing to it vibrate and produce sound waves.
Through the years the design of the
electromagnetic system has been continously improved. In the most
common type of receiver, introduced in the Bell system in 1951, the
diaphragm, consisting of a central cone attached to a ring-shaped
armature, is driven as a piston to obtain efficient response over a
wide frequency range. Telephone receivers are designed to have an
accurate response to tones with frequencies of 350 to 3,500 hertz—a
dynamic range that is narrower than the capabilities of the human
ear but sufficient to reproduce normal speech.
Anti-sidetone circuit. The anti-sidetone circuit is an assemblage of
transformers, resistors, and capacitors that perform a number of
functions. The primary function is to reduce sidetone, which is the
distracting sound of the speaker's own voice coming through the
receiver from the transmitter. The anti-sidetone circuit
accomplishes this reduction by interposing a transformer between the
transmitter circuit and the receiver circuit and by splitting the
transmitter signals along two paths. When the divided signals,
having opposite polarities, meet at the transformer, they almost
entirely cancel each other in crossing to the receiver circuit. The
speech signal coming from the other end of the line, on the other
hand, arrives at the transformer along a single, undivided path and
crosses the transformer unimpeded.
The anti-sidetone circuit also matches the
low electrical impedance of the telephone instrument's circuits to
the higher electrical impedance of the telephone line. Impedance
matching allows a more efficient flow of current through the
system.
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