Man's physical universe

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ELECTROMAGNETIC INDUCTION 511

This may be accomplished by use of an automatic interrupter, which

makes and breaks the current several times a second.

The induction coil consists of a strong electromagnet, made up of a

core surrounded by the primary coil, and a secondary coil usually

wrapped around the primary coil. Induction coils are capable of building

up such high voltages that sparks may be made to jump several

inches through the air. The distance a spark will jump through the air

depends upon the dryness of the air, the shape of the conductors, the

form of gap terminals, and other factors. In ordinary air an electromotive

force of ten to thirty thousand volts is needed to make a spark

jump across one centimeter. One can easily calculate the voltage

required to make a spark jump several inches, using information given

in a previous section.

Induction coils are used where high voltage and

low amperage are needed, as in furnishing a spark for igniting gases in

internal-combustion engines or in operating X-ray tubes.

The Tesla coil is a type of induction coil, designed by an American

electrician, Nicola Tesla, for the production of very high voltage. It

depends on the use of a very rapidly oscillating current in a coil, which

greatly increases the rate at which the magnetic field changes. This

oscillating current, sometimes oscillating with a frequency of a million

times a second, is produced by the alternate charging and discharging

of a condenser. An induction coil is used to charge the condenser.

With the Tesla coil many spectacular experiments can be carried out.

Although the voltage is very high and produces vivid flamelike discharges,

it is quite harmless. One is not hurt by touching one of the

terminals, because the current generated is very small and can do no

harm because of the rapidity of the oscillations, and because the

currents do not penetrate to vital organs.

Transformers Are Induction Coils Operated by Alternating Currents.

Induction coils require interrupters inasmuch as they use the direct

current. Transformers are induction coils used to change the voltage

when employing alternating current. Inasmuch as the direction of the

alternating current changes many times per second, no interrupters

are required in transformers. For many purposes direct current is

required, but alternating current can frequently be used and has

supplanted the direct type in most communities because electric power

can be transmitted more economically by alternating current. The

reason for this is that the voltage in the case of an alternating current

can be raised by a transformer to a point where the loss in transmission

due to resistance is reduced to a minimum; the high voltage can be

lowered by another transformer at the place where the current is to be

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