# Man's physical universe

ELECTROMAGNETIC INDUCTION 517

so that, when the current reverses in the

external circuit are also reversed.

The strength of the electromotive force , and therefore the strength

of the current, will

also be increased byincreasing

the number of coils of wire

to be rotated in the magnetic field

and by increasing the speed of rotation,

which increases the frequency, i.e., cycles

per second.

The number of times the current reverses

in direction depends upon the

number of electromagnets in the stator

and the speed with which the rotor rotates.

Most alternators are built so as to give a

current of 60 cycles (or 120 alternations)

per second.

coil, the connections to the

Fig. 254. The alternating

current from the coil is automatically

reversed by the commutator,

thus changing it to a

direct current.

Motors Convert Electrical Energy into Mechanical Energy.

Faraday made the first very simple motor in 1821. The first commercial

motors were made in the 1840's.

In 1873 a Belgian electrical engineer, Z. T. Gramme, was installing

two electrical generators for the Vienna Industrial Exposition when a

workman accidentally connected the wires from one generator that

was in operation to the other generator that had not yet been installed.

At once it began to rotate. So by accident it was discovered that a

direct-current generator will run as a motor when supplied with a

direct current at the proper voltage.

Any direct-current generator can be converted into a direct-current

motor by supplying the generator with electric current through the

The reason for this is that a mechanical force is exerted by a

brushes.

magnetic field on a conductor carrying a current. Whenever a conductor

carrying a current is placed in a magnetic field in such a direction

that the current flows at right angles to the field,

the conductor

experiences a force at right angles to both the direction of the current

and the magnetic field.

Figure 255 shows a simple motor. In the motor the electricity passes

through the coils of the rotor, causing the core to be magnetized. If

this core is pointed in such a direction that the north and south poles

are opposite the north and south poles of the permanent magnets, it

will be repelled, because like poles repel each other. As it makes half

a turn as a result of this repulsion, the direction of the current

through the commutator is reversed, and the south pole becomes

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