asing the number of contact points and
the wires leading from them, a switch may be made to throw in one or
more pieces of apparatus. This variety of switch is useful in connection
with resistance coils (Index). By joining the ends of the coils with the
points 1, 2, 3, etc., more or less resistance can be easily thrown in by
simply swinging the lever, E, around to the left or right. The uses of
this will be again referred to.

[Illustration: Fig. 25.]

Details. The base of the one shown in Fig. 25 is 4 x 5 x 7/8 in. thick.
The switch, E, is a band of 2 thicknesses of tin 5/8 in. wide. It is
pivoted at F with a screw. To the end of E is fastened a copper wire,
which leads to the upper binding-post, X (App. 46). The apparatus has 5
contact points, marked 1, 2, 3, etc. These consist of brass screws and
copper washers. With F as a center draw the arc of a circle that has a
radius of 4 in. Place the screws 1, 2, etc., along this arc, and about
5/8 in. apart, center to center; that is, the screws are all 4 in. from
F, and are, therefore, in the form of a curve.

The last screw forms a part of the binding-post, Y. Suppose 4 pieces of
apparatus, marked A, B, C, and D, be connected with 1, 2, etc., as
shown. These may be, for example, coils of wire to be used as resistance
coils. If the current enters at X, it will pass along at E and be ready
to leave at Y, as soon as E touches one of the contact points. If E be
placed upon 1, the current will be obliged to pass through all of the
coils, A, B, etc., before it can get to Y. In this case the resistance
will be greatest. If E be now moved on to 2, only A will be cut out, and
the total resistance reduced. By placing E upon 4, but one coil, D, will
be in the circuit. When E is upon 5 the current will pass through the
switch with practically no resistance. This is the principle upon which
current regulators work. (Study resistance in text-book.) When E is in
the position shown in Fig. 25 no current can pass.

CHAPTER V.

BINDING-POSTS AND CONNECTORS.

_57. Binding-Posts_ are used to make connections between two pieces of
apparatus, between two or more wires, between a wire and any apparatus,
etc., etc. They are used simply for convenience, so that the wires can
be quickly fastened or unfastened to the apparatus. There are many ways
of making them at home. The following forms will be found useful and
practical. Although some that are given are really connectors instead o