In order to make a really practical telegraph outfit, it is almost essential that the key and sounder frame be made out of cast iron so as to give the instruments the requisite stiffness. It is easier to use castings than to attempt to bend the parts out of metal.
In order to secure the proper castings it will be necessary to first make a set of "patterns". This is not very difficult.
The key frame is shown in Figure 83 and the Sounder frame in Figure 84. The patterns should be cut out of wood according to the exact shape and dimensions shown in the illustrations. Omit all the holes, however, because no holes should appear in the patterns but only in the finished castings.
[Illustration: FIG. 86—The Sounder Armature.]
The patterns should be made perfectly smooth and be a good, clean job throughout. After they are finished, give them each a coat of shellac and they are ready for the foundry. Almost any iron foundry will make up the castings if supplied with the patterns. If a number of experimenters "chip in" and make up a set of patterns from which several sets of castings are ordered at the same time, the cost will be very slight.
After the castings are secured, they should be cleaned up with a file and drilled according to the illustration in Figures 83 and 84. Several of the holes should be tapped to receive an 8-30 machine screw. Such holes are marked in the illustrations.
The holes not marked "Tap" should be just large enough to allow an 8-32 screw to slip through.
[Illustration: FIG. 87.—Sounder Frame with Lever in Position.]
*The Electro-magnets* are illustrated in Figure 85. The magnet core is made of a piece of 5-16 inch round iron rod an inch and a quarter long. A fibre washer, three-quarters of an inch in diameter is fitted over each end of the core. One end of the core is drilled and tapped to receive an 8-32 screw.
The magnet bobbin, before winding is shown at the left of the illustration and a finished magnet at the right. Two magnets are required. They should both be wound in the same direction with No. 25 single cotton covered wire. The wire should be wound in smooth, even layers and the terminals led out through two small holes in the fibre heads.