Page:Popular Science Monthly Volume 63.djvu/375

Rh also to Professor Braim. In this case, a condenser and inductance are connected in series to the spark balls of an induction coil, and oscillations are set up in this circuit. Accordingly, there are rapid fluctuations of potential at one terminal of the condenser. If to this we connect a long aerial, the length of which has been adjusted to be one quarter of the length of wave corresponding to the frequency, in other words, to make it a quarter wave resonator, then powerful oscillations will be accumulated in this rod. The relation between the height (H) of the aerial and the frequency, is given by the equation $$3 \times 10^{10} = 4nH$$, where n is the frequency of the oscillations and H the height of the aerial in centimeters. The frequency of the oscillations is determined by the capacity (C) and inductance (L) of the condenser circuit, and can be calculated from the formula

That is, the frequency is obtained by dividing into the number 5,000,000, the square root of the product of the capacity in microfarads, and inductance in centimeters, of the condenser circuit. It will be found, on applying these rules, that it is impossible to unite together any aerial of a length obtainable in practise with a condenser circuit of more than a very moderate capacity. It has-been shown that for an aerial two hundred feet in height the corresponding resonating frequency is about one and a quarter million. As we are limited in the amount to which we can reduce the inductance of a discharge circuit, probably to something like a thousand centimeters, a simple calculation shows that the largest capacity we can employ is about a sixtieth of a microfarad. This capacity, even if charged at 60,000 volts, would only contain thirty joules of energy, or about 22.5 foot-pounds, which is a small storage compared to that which can be achieved when we are employing the above described methods, which involve the use of an oscillation transformer. In such a case, however, it is an advantage to employ a spark gap in compressed air, because we can then raise the voltage to a much higher value than in air of ordinary pressure without lengthening the spark so much as to render it non-oscillatory.

When employing methods involving the use of an oscillation transformer, it is possible to use multiple aerials having large capacity, and hence to store up a very large amount of energy in the aerial, which is liberated at each discharge. The most effective arrangement is one