High Frequency Apparatus Page 139

NOTES FOR THE BEGINNER 139 The Condenser and Oscillatory Circuit.-Tracing the course of the high potential current as it leaves the secondary of the transformer, we find that it passes into a device called a condenser. This piece of apparatus consists of a number of sheets of tinfoil separated by plates of mica or glass. The foil sheets are supplied with lugs projecting alternately first from one side and then from the other as the foil and mica plates are assembled. The alternate lugs are soldered together on each side and to these joints the wires from the transformer are fastened. Passing from the condenser we find the current flows through the primary of another transformer, but one without an iron core, and finally across a spark gap and back to the condenser. The condenser acts as a reservoir for the current, which stores up as a charge on the plates until the tension becomes so great that the current leaps across the spark gap in a crashing discharge. This discharge is not composed of a single spark, as appearances would seem to indicate, but it comprises many separate discharges which surge back and forth across the gap with a motion which may be likened to that of a swinging pendulum. When the energy is finally spent the discharge would naturally cease, but during all this time the condenser is again replenishing its supply from the high voltage terminals of the transformer and as soon as one discharge has died away, there is another charge ready to take its place. All of this happens perhaps in the ten thousandth part of a second or less. The oscillatory discharge of the condenser across the gap sets up a current of very high frequency in the circuit, which includes the primary of a second transformer in it, as previously explained. Obviously, therefore, it is only necessary to place within this primary a secondary coil having a suitable number of turns of wire in order to obtain a high frequency current of any desired potential. There