Some contemporary advances in physics — IV

An electrical circuit having a natural oscillation-frequency any-where below 10⁸ 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 electrotechnical. 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 10⁸1 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 10¹¹ 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. This interval is one of the two lacunae in the complete electromagnetic spectrum extending from 10⁴ past 10²⁰ cycles, which were mentioned in the preceding article of this series. Unlike the gap between the ultraviolet and the X-rays, it is not believed to be populated by rays resulting from important processes occurring within the atoms, nor do we know of any other peculiar type of radiation which should be sought within it; and perhaps the bridging of it, when finally and unquestionably achieved, will be held notable chiefly as a feat of experimental technique or a tour de force. On the other hand, so long as the gap remains unspanned, we can hardly dismiss the possibility that something in the order of nature may reserve one range of wavelengths for the “natural” rays resulting from atomic processes, and limit the “artificial” waves generable by electrotechnical means to a distant range which never can be extended to overlap the other.