German silver wire, to give
the same resistance.

NOTE. The number of a wire indicates its diameter; number
30, for example, being always of a definite fixed diameter,
no matter what the material of the wire.

If we wish to avoid loss of current by heating, we use a wire of low
resistance; while if we wish to transform electricity into heat, as in
the electric stove, we choose wire of high resistance, as German
silver wire.

CHAPTER XXXV

HOW ELECTRICITY IS OBTAINED ON A LARGE SCALE

318. The Dynamo. We have learned that cells furnish current as a
result of chemical action, and that the substance usually consumed
within the cell is zinc. Just as coal within the furnace furnishes
heat, so zinc within the cell furnishes electricity. But zinc is a
much more expensive fuel than coal or oil or gas, and to run a large
motor by electricity produced in this way would be very much more
expensive than to run the motor by water or steam. For weak and
infrequent currents such as are used in the electric bell, only small
quantities of zinc are needed, and the expense is small. But for the
production of such powerful currents as are needed to drive trolley
cars, elevators, and huge machinery, enormous quantities of zinc would
be necessary and the cost would be prohibitive. It is safe to say that
electricity would never have been used on a large scale if some less
expensive and more convenient source than zinc had not been found.

319. A New Source of Electricity. It came to most of us as a
surprise that an electric current has magnetic properties and
transforms a coil into a veritable magnet. Perhaps it will not
surprise us now to learn that a magnet in motion has electric
properties and is, in fact, able to produce a current within a wire.
This can be proved as follows:--

[Illustration: FIG. 237.--The motion of a magnet within a coil of wire
produces a current of electricity.]

Attach a closely wound coil to a sensitive galvanometer (Fig. 237);
naturally there is no deflection of the galvanometer needle, because
there is no current in the wire. Now thrust a magnet into the coil.
Immediately there is a deflection of the needle, which indicates that
a current is flowing through the circuit. If the magnet is allowed to
remain at rest within the coil, the needle returns to its zero
position, showing that the current has ceased. Now let the magnet be
withdrawn from the coil; the needle is deflected as before