part. Instead of a simple horseshoe-shaped bar, two shorter bars
riveted into a plate are generally used for electromagnets of this type.
Coils of wire are wound round each bar, and connected so as to form one
continuous whole; but the wire of one coil is wound in the direction
opposite to that of the other. The free end of each goes to a battery
terminal.

In Fig. 51 you will notice that some of the "lines of force" are
deflected through the iron bar A. They pass more easily through iron
than through air; and will choose iron by preference. The attraction
exercised by a magnet on iron may be due to the effort of the lines of
force to shorten their paths. It is evident that the closer A comes to
the poles of the magnet the less will be the distance to be travelled
from one pole to the bar, along it, and back to the other pole.

[Illustration: FIG. 51.--Electro-magnet: A, armature; B, battery.]

Having now considered electricity in three of its forms--static,
current, and rotatory--we will pass to some of its applications.

THE ELECTRIC BELL.

A fit device to begin with is the Electric Bell, which has so largely
replaced wire-pulled bells. These last cause a great deal of trouble
sometimes, since if a wire snaps it may be necessary to take up carpets
and floor-boards to put things right. Their installation is not simple,
for at every corner must be put a crank to alter the direction of the
pull, and the cranks mean increased friction. But when electric wires
have once been properly installed, there should be no need for touching
them for an indefinite period. They can be taken round as many corners
as you wish without losing any of their conductivity, and be placed
wherever is most convenient for examination. One bell may serve a large
number of rooms if an _indicator_ be used to show where the call was
made from, by a card appearing in one of a number of small windows.
Before answering a call, the attendant presses in a button to return the
card to its normal position.

In Fig. 52 we have a diagrammatic view of an electric bell and current.
When the bell-push is pressed in, current flows from the battery to
terminal T^1, round the electro-magnet M, through the pillar P and
flat steel springs S and B, through the platinum-pointed screw, and back
to the battery through the push. The circulation of current magnetizes
M, which attracts the iron armature A attached to the spring S, and
draws the hammer H towards the