Drive a brass headed upholsterer’s nail into the base near the left hand terminal of the resistance so that when the lever is swung around in that direction as far as it will go, the tack will raise the lever up off the resistance wire and break the circuit completely. Two small nails should be driven into the base in such a position that they will limit the swing of the lever and prevent it from moving so far that it passes over the ends of the resistance unit. The rheostat may be connected in a circuit by attaching two wires to the binding posts. It should be placed in series with any device which it is desired to regulate. The carrying capacity is limited to two amperes owing to the fact that a heavier current than this will cause the wire to overheat. Turning the knob so that the lever moves towards the upholsterer’s nail, which raises it up off the wire, increases the amount of resistance in the circuit and cuts down the current, until the lever slides up on the nail when the circuit is broken entirely. Moving the knob in the other direction decreases the resistance and increases the current until the lever has moved as far as it may, when the current will be at a maximum. [Illustration: FIG. 82.—The completed Rheostat.] CHAPTER VIII. HOW TO MAKE A TELEGRAPH KEY AND SOUNDER AND INSTALL A TELEGRAPH LINE. The modern telegraph consists essentially of four things: 1. A battery which produces the electric current. 2. A wire to conduct the current from one point to another. 3. A key or switch for turning the current off and on. 4. An electro-magnetic receiving apparatus, which changes the pulsations of the electric current into sounds. [Illustration: FIG. 83.—Key Frame.] Its operation is not, as many suppose, complicated and difficult to understand, but is quite simple. The key is really a form of switch and is simply a contrivance for controlling the electric current. It consists of a steel lever, pivoted in the centre, and provided with a rubber knob, which the operator grasps lightly with the thumb and forefinger. When the lever is pressed downwards, a platinum point on the under side is brought into contact with another similar point set into a rubber bushing in the base of the key, so that there is no electrical connection between the two points unless the key is pressed down or "closed" as it is usually termed. The receiving device, or "sounder," consists of two electro-magnets mounted on a base under a flat strip or iron, called the armature, which is attracted by the magnetism of the electro-magnets when a current flows through them, and is withdrawn by a spring when no magnetism is produced by the windings. [Illustration: FIG. 84.—Sounder Frame.] The armature is usually mounted on a strip of brass or aluminum called the "lever". The lever strikes against an anvil and produces the clicks which form the dots and dashes of the telegraph alphabet.