Man's physical universe

xanabras

512 MAGNETISM AND ELECTRICITY

used. Thus the transformer has permitted the commercial use of

alternating current. A transformer which increases the voltage is

called a step-up transformer.

Transformers in which the number of turns in the primary is greater

than that in

the secondary decrease the voltage and are called stepdoivn

transformers.

Pig. 247. Distribution transformers.

(Courtesy of the Allis-

Chalmers Manufacturing Company.)

In large transformers, the insulated coils are usually

surrounded by oil to prevent

short circuits and to keep the transformers

cool. (The oil is cooled by

radiators.) The coils in smaller

transformers are frequently packed

in pitch or tar for the same reason.

In distributing current through

a city, it has been found most economical

and safe to transmit the

power at 2000 to 4000 volts. Substations

are generally located outside

the city to transform higher

voltages to the above voltages.

Transformers are also located at

points where this current is distributed

to business houses or groups

of dwellings to transform the voltage

down to 220 or 110 volts. High

voltage is dangerous, but the ordinary

household current of 110 volts is not so hazardous, except when

unusually good contacts are made, such as when one stands on a wet

floor or in a bathtub and touches a poorly insulated electric appliance.

Small transformers used for doorbells or toy railroads step down the

voltage again to 6 or 8 volts.

Transformers which have just one turn in the secondary, using a wire

of large diameter, produce a current of high amperage but low voltage.

Such a current is used in electric welding because it enables sufficient

heat to be obtained at the point of contact of two metals which form

part of the circuit to weld them together; the heat is

the product of the square of the current times the resistance.

proportional to

A Choke Coil Acts by Self-inductance.

A choke coil, such as is used in radios or fluorescent light controls,

consists of a coil of wire through which the current flows. The current

sets up magnetic lines of force that cut the turns of wire in the coil and

thus induce a voltage within the circuit that opposes the original volt-

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