Page:Popular Science Monthly Volume 92.djvu/814

 798

Non- Synchronous Operation

Taking up the non-synchronous dis- charge first, let us imagine that the ro- tating part of either gap is revolved by a direct current motor and that its speed of

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��A diagram of a radio transmitter with a rotary spark gap interposed in the apparatus

rotation has no particular relation to the alternating current supplied to the power transformer in Fig. 41. As the secondary of the transformer charges the condenser, the spark-gap rotating electrodes R, R, are constantly moving toward or away from the fixed electrodes F, F. If the instant of maximum potential of the trans- former should occur while the spark gap contacts are widely separated, no spark would pass and the condenser would dis- charge back into the transformer second- ary on the next half cycle of applied power. If, on the other hand, the spark gap electrodes were quite near together when maximum potential was reached, a spark would pass and a group of radio frequency oscillating currents would be produced in the primary circuit. When the spark gap rotor is driven inde- pendently of the applied alternating current power, i.e., non-synchronously, it is evident that the time a spark will pass (and, in fact, whether or not a spark will pass at all) depends entirely upon chance. The only way to be sure that a spark will pass for each half cycle of alternating current applied to the power transformer is to increase the speed or number of electrodes of the rotary gap so that at least one opportunity for sparking will exist near the maximum voltage portion of each half-cycle. In commercial prac- tice this has usually been accomplished by running the spark gap at a speed which corresponds to approximately GOO sparks (or, more strictly, "opportunities to.

��spark") per second when the supply cur- rent is of 60 cycles per second frequency. Thus in each half-cycle of secondary volt- age there are five instants at which the condenser might discharge across the spark gap, provided only that at each of these times the condenser voltage is higher than the minimum required to break across the shortest spark gap.

How the Condenser Discharge Time Is Varied

How the adjustment of the gap affects the times of sparking may easily be seen by studying Fig. 42. Here the solid curve represents the numerical potential value of the secondary condenser charge, as it is produced by the power transformer and without allowing for effects of with- drawing energy by the spark discharge. The dashed curve above represents the breakdown potentials of the rotating spark gap, and both are drawn for the same successive instants of time. The divisions along the horizontal axis repre- sent time intervals of 1/600 second, and consequently ten of them are contained in two half-cycles or one complete cycle of audio-frequency voltage at 60 cycles per second. The voltage curves are all plotted above the axis, since in this case we are concerned only with the numerical value of the voltage and not at all with its direction — the spark gap will break

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��Curves showing the oj)eration of the non- synchronous spark gap for a wireless set

down whenever the potential rises above a certain approximate value, substan- tially without regard to the direction of potential stress. The voltage is assumed to vary from zero to ten thousand.

Continuing with Fig. 42, the upper dashed curve may be understood by imagining the successive separations be-

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