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Bimetallic coil Carbon granules
Condensations Metal diaphragm
Wires
Contacts
Spring
Mercury in
glass vial

A
Electromagnet (shown expanded from air
Small-current Electromagnet gap of permanent magnet)
relay circuit
Heat pump
Paper cone

FIGURE 6.29 A schematic of a relay circuit. The mercury
vial turns as changes in temperature expand or contract the coil, Permanent magnet
moving the mercury and making or breaking contact with the relay Condensations
circuit. When the mercury moves to close the relay circuit, a small
current activates the electromagnet, which closes the contacts on
the large-current circuit. B

FIGURE 6.30 (A) Sound waves are converted into a changing
electrical current in a telephone. (B) Changing electrical current
glass tube. When the temperature changes enough to tip the can be changed to sound waves in a speaker by the action of an

glass tube, the mercury flows to the bottom end, which makes electromagnet pushing and pulling on a permanent magnet. The
or breaks contact with the two wires, closing or opening the cir- electromagnet is attached to a stiff paper cone or some other mate-
cuit. When contact is made, a weak current activates an electro- rial that makes sound waves as it moves in and out.
magnetic switch, which closes the circuit on the large-current

furnace or heat pump motor. The loudspeaker in a radio or stereo system works from
A solenoid is a coil of wire with a current. Some solenoids changes in an electric current in a similar way, attracting and
have a spring-loaded movable piece of iron inside. When a cur- repelling a permanent magnet attached to the speaker cone. You
rent flows in such a coil, the iron is pulled into the coil by the can see the speaker cone in a large speaker moving back and

magnetic field, and the spring returns the iron when the cur- forth as it creates condensations and rarefactions.

rent is turned off . This device could be utilized to open a water
valve, turning the hot or cold water on in a washing machine or Electric Motors
dishwasher, for example. Solenoids are also used as mechanical
An electric motor is an electromagnetic device that converts
switches on VCRs, automobile starters, and signaling devices
electrical energy to mechanical energy. Basically, a motor has
such as bells and buzzers.
two working parts, a stationary electromagnet called a fi eld
magnet and a cylindrical, movable electromagnet called an
Telephones and Loudspeakers armature. The armature is on an axle and rotates in the magnetic

The mouthpiece of a typical telephone contains a cylinder of field of the field magnet. The axle turns fan blades, compressors,

carbon granules with a thin metal diaphragm facing the front. drills, pulleys, or other devices that do mechanical work.

When someone speaks into the telephone, the diaphragm Different designs of electric motors are used for various
moves in and out with the condensations and rarefactions of the applications, but the simple demonstration motor shown in
sound wave (Figure 6.30). This movement alternately compacts Figure 6.31 can be used as an example of the basic operating prin-

and loosens the carbon granules, increasing and decreasing the ciple. Both the field coil and the armature are connected to an

electric current that increases and decreases with the condensa- electric current. The armature turns, and it receives the current

tions and rarefactions of the sound waves. through a commutator and brushes. The brushes are contacts that

The moving electric current is fed to the earphone part of a brush against the commutator as it rotates, maintaining contact.

telephone at another location. The current runs through a coil When the current is turned on, the field coil and the armature

of wire that attracts and repels a permanent magnet attached to become electromagnets, and the unlike poles attract, rotating the
a speaker cone. When repelled forward, the speaker cone makes armature. If the current is dc, the armature will turn no farther,
a condensation, and when attracted back, the cone makes a stopping as it does in a galvanometer. But the commutator has

rarefaction. The overall result is a series of condensations and insulated segments so when it turns halfway, the commutator seg-
rarefactions that, through the changing electric current, accurately ments switch brushes and there is a current through the armature
match the sounds made by the other person. in the opposite direction. This switches the armature poles, which

160 CHAPTER 6 Electricity 6-22