CHAPTER XI. AERIAL SWITCHES AND ANCHOR GAPS.
Some form of switch for quickly connecting the aerial and ground to either the transmitting or receiving apparatus is necessary.
Low powered stations using an induction coil as a transmitter will find a 250-volt double pole, double throw porcelain base switch to be suitable. Such a switch is connected up according to the diagram shown in Fig. 70. As clearly illustrated, the receiving apparatus is in use when the switch is thrown to the right, and the transmitter is ready for operation when the switch is in the left-hand position. The ease and speed with which such a switch may be thrown from right to left or vice versa may be much facilitated by fitting it with a larger and longer handle than the one usually provided.
Fig. 70. Connections for Aerial Switch.
An ordinary double pole, double throw switch should be mounted on the wall within easy reach of the operator, while the two types of switch about to be described are best situated directly on the operating table alongside of the transmitting key.
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.
Fig. 73. Details of Switch Parts.
Some may prefer to make the complete switch, and in that case the dimensions given in Fig. 73 may be of aid. The knife blades are 6 inches long, 5/8 inch wide and 1/8 inch thick. Three are required. A 1/8-inch hole is bored 3/8 inch from one end, and the other end is cut out in the shape shown in the illustration. An insulating bar 5 x 3/4 x 5/8 inches is made of hard rubber and three notches are cut in the back face, one directly in the center and one 2 inches on either side.
Fig. 74. Method of Fastening Knife.
Three 1/8-inch holes are bored one in the center of each groove and at right angles to it. An 8-32 machine screw 3/4 inch long is placed in the notch in the end of the knife blade and the screw passed through the hole in the insulating bar. The blade may then be clamped tightly in place by putting a nut on the end of the screw. The screw which holds the middle knife blade in place is longer than the other two so that a handle may be fitted on.
The bearings are made the same size and shape as the contacts illustrated in Fig. 72, with the exception that a 1/8-inch hole is bored through near the top so that a small bolt may be slipped in to hold the knife blades in place.
The base of the switch measures 8 x 9 x 3/4 inches. The parts are assembled as in Fig. 71.
Another type of aerial switch called a "T" switch is illustrated in the same figure. A piece of 3/4-inch hard rubber is sawed in the shape of a T, 3 1/2 inches high and 6 1/2 inches wide at the top. The upper contacts are mounted on arms of the "T" and connected to binding posts placed immediately above them, A slot, 1/2 inch wide and 3 inches long, sawed in the center of the leg of the T, permits the middle knife blade to move without obstruction.
Fig. 75. "T" Aerial Switch.
A double pole, double throw switch may be easily adapted to form a "T" switch. Two knives are cut from sheet brass, of the same shape as those already on the switch but twice as long. The contacts at one end of the porcelain base are removed and the bearings of the knife blade fastened there in their stead. This forms a single throw, double pole switch having knife blades twice the length of the original double pole, double throw switch. A "T" shaped support for the upper contacts may be sawed out of 1/2-inch hard rubber and smoothed up with a file. The support is fastened upright by means of two screws which pass through holes in the center of the base where the knife bearings originally were. The two contacts which were removed from the base are fastened to the support directly over the knife blades so that they make a good contact when the switch is up. Two binding posts mounted above the contacts make connection with them.
If desirable the switch may be fitted with a third knife blade and contact so that when the switch is up in position for receiving, the primary circuit of the induction coil or small transformer is open.
It is obvious that the dimensions, etc., need not be the same as those given here but may be adapted to suit the size and design of the switch which is to be altered. The knives should always be as long as convenient and the contacts separated by a wide space.
Fig. 76. "United" Wireless Lightning Switch.
Fig. 77. Shoemaker Tuning Coil and Aerial Switch.
Figs. 76 and 77 illustrate the aerial switches used in the installations of the United Wireless Telegraph Company. These illustrations give a good idea of the long knife blades employed. The lightning switch is fitted with a micrometer spark gap on top of the switch which is in service when the handle is up. The small spark gap affords a path for the lightning to jump into the ground, but cannot be bridged by the receiving currents.
Anchor Gaps.—When a loop aerial system is employed for both transmitting and receiving, a device known as an anchor gap becomes necessary.
Fig. 78. "United" Wireless Anchor Gaps.
The construction and connection of a simple anchor gap are shown in Fig. 79. A ring 1 3/4 inches outside and 1 1/4 inches inside is cut out of 5/8-inch hard rubber or fiber and smoothed up. Three 1/8-inch holes are bored in the periphery of the ring at 120 degrees to each other. The holes are threaded with a 10-24 tap. Three small binding posts are each fitted with a 3/16-inch brass rod 7/8 inch long, having a 10-24 thread so that they may be screwed into the holes in the ring.
Fig. 79. Anchor Gap.
The gap between the ends of the two rods connected to the aerial and the receiving apparatus is adjusted until it is about 1/8 inch long. The third point is screwed in until it all but touches the other two.