High Frequency Apparatus Page 13

HOW THE CURRENT IS PRODUCED 13 have induced in it a high frequency current similar to that oscillating in the primary circuit. By means of a suitable adjustment of the ratio existing between the turns in the two coils, the potential delivered at the secondary terminals may be increased practically at will. The most familiar use of the arc as a high frequency current generator is in the field of radio telephony. The purity of the wave generated by the arc renders it particularly well adapted to this use. For purposes of demonstration, however, the arc generator is not capable of delivering a sufficiently large output. With all due respect to the method in the work for which it is best adapted, we shall therefore recommend that the experimenter discard it, using in its stead, the condenser discharge form of generator, a detailed description of which follows. Condenser Discharge Generator.—It is assumed that the average reader of this book will be familiar with the elementary principles of wireless apparatus. Granting this, it is, of course, reasonable to believe that such readers will understand how an oscillatory current is set up in a circuit comprising an inductance or coil of wire, a capacity or condenser, and a spark gap. The condenser is charged with a high tension current from any convenient source such as a transformer or induction coil, and when the potential stored up in the condenser reaches a critical value, the air in the gap between the spark gap electrodes can no longer stand the strain, and the condenser discharges across the gap in a succession of crashing sparks. As the current from the condenser crosses the gap in one direction, it literally over-reaches itself just as a pendulum swings past the neutral point when given a push with the hand. When the first rush of current passes in one direction, a reversal of the cycle occurs and a second rush in the opposite direction is effected. This operation is repeated many thou