[Illustration: Fig. 160. Methods of Preventing "Kick Back."]
The first method shows an ordinary pressed telephone condenser of about two microfarads capacity connected directly across the A. C. mains near the transformer terminals, in series with two 6-ampere fuses. The condenser is shunted by a small spark gap made of needle points with a very small space, about .005 of an inch, between them.
The second method is an elaboration of the first and shows two sets of condensers in series with fuses and bridged by spark gaps with a "ground" through a third condenser connected between. This second method is the best and is often used to prevent delicate instruments, such as a voltmeter, from the effects of "kick back."
A proper "kick-back" preventer is part of the Fire Insurance Underwriters requirements for a wireless telegraph station.
A variometer is a tuning device in which two coils of wire are placed in series and connected so that the turns can be made to oppose one another. One coil is movable and by turning it the currents flowing through the adjacent coils oppose each other and decrease the self-induction of the whole and consequently the period of the circuit.
The accompanying illustration shows the constructive details of an efficient form of variometer.
The coils are wound around two cardboard cylinders. One cylinder is 6 inches in diameter and the other 5 inches. Both are 2 inches long. The large cylinder is wound with twenty-five turns of No. 22 B. S. gauge single silk covered wire wound in two sections so as to leave a space 3/8 inch wide in the center. The small cylinder is wound in the same manner with the same size of wire but contains thirty turns so as to make the length of wire in each of the coils practically equal.
The space in the center of each of the coils allows a supporting rod to pass through without interfering with the wire. The shaft is a piece of 5/16 _brass_ rod about 7 1/2 inches long. The upper end is provided with a knob and a pointer. The large coil is mounted on the under side of a wooden cover made to fit a containing case of the proper size to receive the variometer and of a style similar to that ordinarily used to enclose a variable condenser.