In Section 299 we learned that substances differ very greatly in the resistance which they offer to electricity, and so it will not surprise us to learn that while it takes 300 feet of iron telegraph wire to give 1 ohm of resistance, it takes but 39 feet of number 24 copper wire, and but 2.2 feet of number 24 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, but the deflection is in the opposite direction, showing that a current exists, but that it flows in the opposite direction.  We learn, therefore, that a current may be induced in a coil by moving a magnet back and forth within the coil, but that a magnet at rest within the coil has no such influence.