small loss in heat. In an ordinary
16 candlepower carbon lamp, the resistance of this filament is 220
ohms. Therefore the amount of current that a 110-volt generator can
force through that filament is 110/220, or 1/2 ampere.

[Illustration: Armature and field coils of a direct current dynamo]

One hundred lamps would provide 100 paths of 220 ohms resistance each
to carry current, and the amount required to light 100 such lamps
would be 100 x 1/2 or 50 amperes. Every electrical device--a lamp, a
stove, an iron, a motor, etc.,--must, by regulations of the Fire
Underwriters' Board be plainly marked with the voltage of the current
for which it is designed and the amount of current it will consume.
This is usually done by indicating its capacity in watts, which as we
have seen, means volts times amperes, and from this one can figure
ohms, by the above formulas.

_A Short Circuit_

We said a few paragraphs back that under no conditions must two bare
wires leading from electric mains be permitted to touch each other,
without some form of resistance being interposed in the form of lamps,
or other devices. Let us see what would happen if two such bare wires
did touch each other. Our dynamo as we discover by reading its plate,
is rated to deliver 50 amperes, let us say, at 110 volts pressure.
Modern dynamos are rated liberally, and can stand 100% overload for
short periods of time, without dangerous overheating. Let us say that
the mains conveying current from the armature to the switchboard are
five feet long, and of No. 2 B. & S. gauge copper wire, a size which
will carry 50 amperes without heating appreciably. The resistance of
this 10 feet of No. 2 copper wire, is, as we find by consulting a wire
table, .001560 ohms. If we touch the ends of these two five-foot wires
together, we instantly open a clear path for the flow of electric
current, limited only by the carrying capacity of the wire and the
back pressure of .001560 ohms resistance. Using Ohm's Law, C equals E
divided by R, we find that C (amperes) equals 110/.001560 or _70,515
amperes_!

[Illustration: A direct current dynamo]

Unless this dynamo were properly protected, the effect of such a
catastrophe would be immediate and probably irreparable. In effect, it
would be suddenly exerting a force of nearly 10,000 horsepower against
the little 10 horsepower water wheel that is driving this dynamo. The
mildest thing that could happen would be to melt the feed-wir