*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. *The Sounder Armature* is a piece of steel or iron rod, five-sixteenths of an inch square and three and three-eighths inches long. It serves both as the armature and the sounder lever. The exact location of the holes is shown in Figure 86. The holes "h" and "i" should be tapped to receive an 8-32 screw. The hole "j" should pass a screw of the same size. This screw serves as the pivot for the armature. Figure 87 shows how the sounder frame should appear when the lever is in position. The screw A controls the distance which the armature moves on its upward stroke. The screw B strikes the "anvil" on the downward stroke and controls the distance between the armature and the electro-magnets. [Illustration: FIG. 88.—Top View of Completed Instrument] The armature is pushed upwards and held away from the magnets against the screw A by a small spiral spring. The lower end of the spring sets in the hole "C" in the sounder frame and the upper end rests against the end of the screw C so that the tension of the spring can be regulated by moving the screw up and down. The armature should be free to move up and down on the pivot screw but should not move sideways. *The Base* of the instrument is a rectangular piece of hardwood, four and one-half inches long, three and three-quarter inches wide and one-half inch thick. The sounder is mounted on the base in the position shown in Figure 88, which is a top view of the complete instrument. Two long 8-32 machine screws pass upwards through the wooden base, through the holes "e" and "f" in the sounder frame and into the bottom of the magnets, thus serving not only to hold the magnets in position but also the sounder frame to the base. [Illustration: FIG. 89.—Side View of Key.] Figure 89 is a side view of the completed key. The key lever and the circuit closing lever are illustrated in Figure 90. The lever is four and three-eighths inches long and five-sixteenths of an inch wide. It should be made from spring brass. The circuit closing lever is a strip of brass of the same width. All the holes in both levers, with the exception of the centre hole in the key lever should be large enough to pass an 8-32 screw. The centre hole should be tapped to receive an 8-32 screw.