Man's physical universe

xanabras

504 MAGNETISM AND ELECTRICITY

A Coil of Wire Carrying an Electric Current Behaves Like a Bar Magnet.

When a wire carrying a current is bent into the shape of a loop, all

the lines of force outside the loop are pointing in one direction, and

the same is true of those inside the loop. There is thus a strong mag-

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Fig. 242.

A coil of wire conducting an electric current behaves like a bar magnet

as shown by the compass needles.

netic field within the loop. If many loops or turns are added, the magnetic

field becomes still stronger. Such a coil of wire carrying a current

behaves like a bar magnet.

The magnetic field through a coil of wire can be very greatly

strengthened by placing a core of soft iron in the center. Because of

Fig. 243. The

magnetic field of

a helix.

its high permeability to the magnetic lines of force, a great number of

lines of force pass through the iron core and make of it a powerful magnet.

Such an electromagnet, as it is called, acts as a

permanent magnet, but only so long as the current

is flowing through the coil; its polarity depends on

the direction of flow of the current. The strength

of the electromagnet is proportional to the amount

of current flowing through the wire and also to the

number of turns in the coil. Electromagnets have

many applications; doorbells, telegraph sounders,

magnetic brakes, sensitive relays, lifting magnets,

and many other devices are operated by electromagnets.

A coil of wire carrying an electric current is called

a solenoid. Solenoids are used in industry to operate

many automatic devices. If a coil of wire is placed

in a vertical position, an iron core, which is held

within the coil by the magnetic field as long as the

coil is carrying an electric current, will drop through

the coil when the circuit is broken. Such a device can be used to

lock and unlock doors.

The ordinary electric doorbell consists of a clapper which strikes

the bell when the iron strip to which it is attached is attracted to an

electromagnet.

A spring draws the iron piece back when the current

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