Some Contemporary Advances in Physics-IV

N electrical circuit having a natural oscillation-frequency any- where below 10s can be constructed by anyone with suitable condensers, inductance-coils, and a few feet of wire at his disposal. It can be set into oscillation by abruptly closing it when the condenser is charged, by coupling it to an audion, or otherwise; and the waves which it radiates while oscillating can be detected and measured, at least when the frequency exceeds 104. Thus it is possible to generate perceptible electromagnetic waves of frequencies up to 108, and hence of wavelengths down to 3 metres, by methods that may be called eleclrotechnical. Waves shorter than 300 cm., frequencies higher than 108 cycles, are not easily produced by any such method; for if one uses excessively small condensers and inductance-coils in the hope of forcing the circuit-frequency much past 108, or even omits coils and condensers altogether, it is found that the auxiliary apparatus, the audion, even the wires of the circuit themselves, possess capacities and inductances which can not be annulled and which hold the oscillation-frequency down. By devising oscillating systems which have scarcely any outward resemblance to the circuits of familiar experience (although a formal analogy can be established) Hertz and his successors generated electromagnetic waves of frequencies up to 10 u and wavelengths down to 3 mm. Beyond a certain gap there commences, near frequency 1012 and wavelength 0.3 mm., the far-flung spectrum of rays emitted by molecules and atoms.