rical work. He must make a battery for the
production of current, a pair of electro-magnets to be acted upon by the
current, a switch to control it, and, finally, he must learn how to
connect it up so that it may be operated not only from one, but from two
or more push buttons.

[Illustration: _Fig. 48._ PUSH BUTTON]

HOW MADE.--In Fig. 49 is shown an electric bell, as usually constructed,
so modified as to show the structure at a glance, with its connections.
A is the base, B, B' the binding posts for the wires, C, C the
electro-magnets, C' the bracket for holding the magnets, D the armature,
E the thin spring which connects the armature with the post F, G the
clapper arm, H the bell, I the adjusting screw on the post J, K the wire
lead from the binding post B to the first magnet, L the wire which
connects the two magnets, M the wire which runs from the second magnet
to the post J, and N a wire leading from the armature post to the
binding post B'.

[Illustration: _Fig. 49._ ELECTRIC BELL]

The principle of the electric bell is this: In looking at Fig. 49, you
will note that the armature bar D is held against the end of the
adjusting screw by the small spring E. When a current is turned on, it
passes through the connections and conduits as follows: Wire K to the
magnets, wire M to the binding post J, and set screw I, then through the
armature to the post F, and from post F to the binding post B'.

[Illustration: _Fig. 50._ ARMATURE OF ELECTRIC BELL]

ELECTRIC BELL--HOW OPERATED.--The moment a current passes through the
magnets (C, C), the core is magnetized, and the result is that the
armature (D) is attracted to the magnets, as shown by the dotted lines
(O), when the clapper strikes the bell. But when the armature moves over
to the magnet, the connection is broken between the screw (I) and
armature (D), so that the cores of the magnets are demagnetized and lose
their pull, and the spring (E) succeeds in drawing back the armature.
This operation of vibrating the armature is repeated with great
rapidity, alternately breaking and re-establishing the circuit, by the
action of the current.

In making the bell, you must observe one thing, the binding posts (B,
B') must be insulated from each other, and the post J, or the post F,
should also be insulated from the base. For convenience we show the post
F insulated, so as to necessitate the use of wire (N) from post (F) to
binding post (B').

The foregoing assumes th