All insulated parts of an aerial switch such as the base, insulating bar, handle, etc., should be made of some good insulating material, as porcelain, hard rubber or ebonite. All insulation which is used in power switches such as slate, wood, marble, etc., is worthless and should not be used.
If a loop aerial system is used, a switch is necessary, which will break the primary transmitting current so that in case of an accidental touch of the key while receiving, the high voltage discharge will not pass across the anchor gap into the receptor and badly damage the same or shock the operator. The connections of such a switch and the loop aerial are shown in the wiring diagram of the De Forest system.
Fig. 71. Aerial Switches.
A switch may be easily constructed from a 50 ampere, single throw, triple pole, fuseless power switch. If it is mounted on a slate base remove it and set it up on a base of the same size made of fiber or porcelain. Flat unglazed tiles of various sizes are obtainable from tile setters at a very low cost. They may be easily bored with an ordinary steel twist drill. Such tiles are excellent insulators when not exposed to moisture and are useful for bases, etc., in many cases. A heavy coat of shellac varnish will make the tile impervious to moisture.
A hard rubber rod, S, 3 inches long and 5/8 inch diameter, is mounted on both sides of the base in alignment with the contacts. A strip of hard rubber, Y, 1 inch wide, 1/2 inch thick and 6 1/2 inches long is fitted with two contacts similar to those mounted on the base of the switch. The contacts are mounted at a distance apart equal to the distance between the outside knife blades of the switch. A 1/8-inch hole is bored through the strip at both ends, through the axis of the rods along their entire length and through the base. A piece of brass rod 4 inches long and threaded at both ends with an 8-32 die is passed through the holes and yoke, so that the rods may be held firmly to the base and the strip to the tops of the rods by two nuts screwed on the ends of the brass rod.
Fig. 72. Detail of Contacts.
The upper contacts should be directly over the two outside lower ones. They are made from a strip of brass 1/2 inch wide, 3 1/2 inches long and 1/16 inch thick (bent as in Fig. 72). A binding post is mounted on the yoke directly above and connecting with each one of the contacts.
The middle pole of the switch is connected to the primary of the coil or transformer, so that when the switch is down, the primary circuit is completed, and when it is up it is broken.