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.