[Illustration: FIG. 81.—The Lever, Knob, Binding Posts, etc.]
The completed rheostat is shown in Figure 82. The right hand terminal of the German Silver wire is connected to the right hand binding post. The other binding post is connected to the washer around the lever shaft on the under side of the base. The wires should be laid in grooves cut in the bottom of the base for that purpose.
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.