Man's physical universe

xanabras

522 MAGNETISM AND ELECTRICITY

ignited, thus causing a fire, when an excessive current is passed through

the wire. For high-power currents, wires of larger diameter must

be used if voltage is kept low, in order to prevent their excessive

heating.

Excessive currents pass through wires when they are short-circuited,

and many fires in automobiles and homes are caused in this way. A

short circuit is caused by connecting two wires through such a low

resistance that the current flowing through the wire becomes very

great, causing the wires themselves to become hot enough to start a

fire or even melt — all because of their own resistance.

In order to avoid such fires, fuses are introduced into the circuit. A

fuse is a wire or strip of some metal or alloy which has a low meltingpoint

and a comparatively high resistance, enclosed in a fireproof box.

As the current increases, the fuse is melted and the circuit is broken

before the copper wires get hot enough to cause any damage. Fuses

are rated according to the current strength at which they will

melt.

Thus, a 10-ampere fuse will allow currents up to 10 amperes to pass

but will melt when the current exceeds that strength. All delicate and

expensive measuring instruments should be protected from being

burned out by use of the appropriate fuses in the circuits in which they

are employed. The practice of replacing a burned-out fuse with a

penny or some other conductor which will not burn out is dangerous

because, although it does permit current to flow once more, it offers no

protection and certainly no remedy for the situation that caused the

fuse to blow out.

A short-circuited line is not the only cause of fuses blowing out.

too many electric appliances are connected in a line that is not designed

to carry such a hea\^ load, the fuse will burn out. Large circuitbreakers

are used in power substations to protect the equipment

against very strong currents that may be produced by lightning or

power-system faults. Lightning is the most common source of failure

of power during thunderstorms. Sometimes the current is restored

at once by merely replacing the fuse. At other times, damage done

before the fuse had time to melt may have to be repaired before the

power can be restored.

The Development of Electric Lamps Is a Record of Continuous Research.

The first type of electric light to be used was the arc lamp, based

upon Sir Humphry Davy's discovery in 1801 that a brilliant flame, or

arc, can be produced by passing a current through a gap between the

adjacent ends of two carbon rods. This discovery was of little value

until the electric generator made possible the economical production

If

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