which plate is hinged a swinging switch blade (G),
the free end of which is adapted to engage with the plates (A, B).

[Illustration: _Fig. 45._ SLIDING SWITCH]

SLIDING SWITCH.--This sliding switch form may have the contact plates
(A, B and C, Fig. 46) circularly arranged and any number may be located
on the base, so they may be engaged by a single switching lever (H). It
is the form usually adopted for rheostats.

REVERSING SWITCH.--A reversing switch is shown in Fig. 47. The base has
two plates (A, B) at one end, to which the parallel switch bars (C, D)
are hinged. The other end of the base has three contact plates (E, F, G)
to engage the swinging switch bars, these latter being at such distance
apart that they will engage with the middle and one of the outer plates.
The inlet wires, positive and negative, are attached to the plates (A,
B, respectively), and one of the outlet wires (H) is attached to the
middle contact plate (F), while the other wire is connected up with both
of the outside plates. When the switch bars (C, D) are thrown to the
left so as to be in contact with E, F, the outside plate (E) and the
middle plate (F) will be positive and negative, respectively; but when
the switch is thrown to the right, as shown in the figure, plate F
becomes positive and plate E negative, as shown.

[Illustration: _Fig. 46._ RHEOSTAT FORM OF SWITCH]

PUSH BUTTONS.--A push button is but a modified structure of a switch,
and they are serviceable because they are operating, or the circuit is
formed only while the finger is on the button.

[Illustration: _Fig. 47._ REVERSING SWITCH]

In its simplest form (Fig. 48) the push button has merely a circular
base (A) of insulating material, and near one margin, on the flat side,
is a rectangular plate (B), intended to serve as a contact plate as well
as a means for attaching one of the wires thereto. In line with this
plate is a spring finger (C), bent upwardly so that it is normally out
of contact with the plate (B), its end being held by a binding screw
(D). To effect contact, the spring end of the finger (C) is pressed
against the bar (B), as at E. This is enclosed in a suitable casing,
such as will readily suggest itself to the novice.

ELECTRIC BELL.--One of the first things the boy wants to make, and one
which is also an interesting piece of work, is an electric bell.

To make this he will be brought, experimentally, in touch with several
important features in elect