*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.
The key lever is fastened to the frame by means of a round headed 8-32 machine screw. The screw should be tightened so that the lever cannot move from side to side. The circuit closing lever is pivoted to the back end so that it can swing. The key lever should be fitted with a regular key knob and the circuit closing lever with a small knob such as is generally used for the purpose. Both of these styles of knobs can be purchased from almost any electrical supply house.
The contact points are best made of silver. Silver makes a better contact metal than almost anything else, with the exception of platinum. If silver cannot be obtained a piece of large german-silver wire may be used.
[Illustration: FIG. 90.—Key and Circuit Closing Levers.]
One contact is soldered to the under side of the key lever. The other contact should be mounted on the head of a screw and pass through a hole in the iron key frame and through the wooden base underneath. This lower contact should be directly underneath the upper contact and be thoroughly insulated from the base. Several thicknesses of well shellaced paper will form a first class insulating medium. The length of the contacts should be such that there is a distance of about 3-32 of an inch between them when the lever is horizontal.