The rotating member is cast from an alloy of equal parts of zinc and aluminum. It is necessary to first make a wooden pattern from which the casting may be made. The details of the wheel are shown in Fig. 158. The casting must be placed in a lathe chuck and turned true. It is mounted on a hard rubber disk 2 7/8 inches in diameter and 1/4 of an inch thick. The disk serves to insulate the revolving electrodes from the motor shaft. The "rotor" is mounted upon the shaft by means of a small brass bushing which passes through the center of the disk.
Fig. 158. Details of Revolving Parts of Rotary Gap.
The motor must be well built and capable of running at high speed. A "Juno" motor will be found very satisfactory. When running free its speed is about 4500 r.p.m. With the rotor in place the speed is about 3600 r.p.m.
The motor should be mounted on a heavy marble base capable of absorbing any little vibration that the gap may be subject to when running at high speed.
The stationary electrodes are made in the same manner as those for an ordinary gap and consists of two flanged zinc electrodes mounted upon threaded brass rods supported by two hexagonal standards. The axis of the electrodes should be the same height above the base as that of the motor shaft.
The rotor should be carefully balanced so that it is practically free from vibration by boring small holes in the back face so as to make the weight on opposite sides equal.
Fig. 159. Details of Rotary Gap.
The motor may be driven by a battery or from the same source that supplies the transformer, in series with two or three suitable lamps. A motor wound to run directly from the 110-volt line or a higher potential must have its fields wound with very fine wire and is apt to give trouble through "burn-outs," due to "kick back." When the motor is operated on batteries or is wound for running in series with a lamp the danger is lessened.