High Frequency Apparatus Page 196

196 Fig 9 HIGH FREQUENCY APPARATUS Fig 10 Fig 11 Mole to fit motor shaft Fig 12 Figs. 9 to 12 inclusive.-The rotary spark gap complete and in detail upon a boss of brass which is threaded into the fibre and riveted over on the end to prevent its working loose. The brass piece is drilled with a hole of a diameter to take the motor shaft and a set screw provides a means for securing the boss on the shaft. The final cut which brings the fibre hub to the diameter of the hole in the aluminum disc should be made after the hub has been cut roughly to size and mounted upon the brass boss which is in turn mounted upon an arbor in the lathe. This will insure absolute truth in running. The aluminum disc should next be divided with 12 radial lines running from the center and crossing the 9-inch circle at 12 equi-distant points. At each point a clearance hole for a No. 10-24 machine screw is drilled. Twelve cylinders each 3/4-inch long should be cut from a piece of nickelsteel rod 3/4-inch in diameter and each cylinder drilled and tapped in one end for a, No. 10-24 screw. The cylinders may then be secured in position on the aluminum rotor and the disc mounted upon its hub of fibre. It is secured thereon with 6 rivets which may be brass escutcheon pins inserted in snugly fitting holes drilled through aluminum and fibre. The heading over should be done very carefully and on the aluminum side. The rotor, which is now finished, may then be mounted upon the arbor in the lathe in order to