[Illustration: Fig. 95.—Simple Switches. _A_, Single-Point Switch. _B_, Two-Point Switch. _C_, Three-Point Switch. _D_, Five-Point Switch. _E_, Lever with End Rolled up to form Handle. _F_, Lever with Handle made from part of a Spool.] The first one shown (_A_) has one contact, formed by driving a brass-headed tack through a small strip of copper or brass. The movable arm is a strip of copper or brass, rolled up to form a handle at one end. The other end is pivoted with a brass screw. The brass screw passes through a small strip of copper or brass having a binding-post mounted on the end. A small copper washer should be placed between the movable arm and the copper strip to make the switch work more easily. A somewhat similar switch is shown by _B_ in the same illustration, only in this case a handle made from half of a spool is used, instead of rolling up the end of the arm. The other illustrations show how the same method of construction may be applied to make switches having more than one "point" or contact. No dimensions have been given for constructing these switches, because it is doubtless easier for the young experimenter to use materials which he may have at hand, and construct a switch of his own proportions. Only one suggestion is necessary, and that is to bevel the under edges of the arm with a file, so that it will slip over the head of the brass tack more easily. The switches shown in Figure 96 are capable of carrying heavier currents than those just described, and more nearly approach the type used on lighting and power switchboards. The base may be made of wood, but preferably should be made of some insulating substance such as fiber or slate. [Illustration: Fig. 96.—Knife Switches.] The patterns for the metal parts are shown in Figure 97. These are cut from heavy sheet-brass or sheet-copper, and then bent into shape with a pair of flat-nosed pliers. The handle of the single-pole switch is driven on over the metal tongue. The double-pole switch is almost a duplicate of the single-pole type, but has two sets of levers and contacts, actuated by the handle, in place of one. [Illustration: Fig. 97.—Metal Parts for the Knife Switches.] The ends of the blades to which the handle is connected are turned over at right angles and a hard-wood cross-bar fastened between the ends. The handle is fastened to the center of the cross-bar. After the switch is assembled, bend the various parts until they "line up" that is, are in proper position in respect to each other, so that the blades will drop into the contacts without bringing pressure to bear on either one side or the other of the handle in order to force the blades into line. Fuses Fuses are used to prevent electrical instruments and wires from damage due to too much current flowing through. When an electric current passes through a resistance it produces _heat_.