Fig. 93.
The Strap, D, is made of tin. It is 4 × ½ in. before bending up the right end a little. It is fastened to the base by the screw, F, and by the other binding-post, Y. Its right end is raised enough to allow the arm, E, to pass under it, but it must press down well upon E when E is forced toward F.
The Swinging Arm or Switch, E, is also made of tin, and measures, finished, 4½ × ½ in. Its front end should be bent up a little for convenience in handling it. (See Fig. 92.) E is pivoted at G by a screw, which also binds the wire, C W. Fig. 24 shows another way to make the pivot and connection.
194. Operation. See Fig. 99 for the details of the connections of a home-made telegraph line. When you are using the line and telegraphing to your friend, the switch, E, of your instrument must be open, as in Fig. 93, and the corresponding switch on his instrument must be closed; that is, the circuit must be opened and closed at but one place at a time. As soon as you have finished, your switch must be closed. He will open his and proceed. When you have both finished, both switches must be closed. If your friend left his switch open, you could not call him over the line, as no current could pass into his sounder.
195. Batteries. As the circuit has to be left closed for hours and perhaps days at a time, so that either operator can call the other, a closed-circuit battery is necessary. ([See App. 9].) A dry cell, Leclanché, or other open-circuit cell would not be at all suitable for a telegraph line, as it would soon polarize. Large Daniel cells, which are 2–fluid cells like [App. 7], or gravity cells ([App. 9]) are the best for your line.
196. Telegraph Sounder. Fig. 94. The wood-work consists of 2 parts; the base, B, is 6 × 4 × ¾ in., and the back, A, is 6 × 5 × ½ in. A is nailed or screwed to B.
