Short Electric Waves

Abstract

Extension of electric wave spectrum to wave-length of 1.8 mm.—As a result of improved apparatus and technique, the electric wave spectrum has recently been extended two octaves nearer to the long wave limit of the heat spectrum. Improved Hertzian oscillators were made with pairs of tungsten cylinders 0.5 to 0.2 mm in diameter and 5 to 0.2 mm long, sealed into glass. Increased efficiency was obtained by use of jets of compressed air for the auxiliary gaps and of kerosene for the main gap. As the length of oscillator decreases from 10 to 0.4 mm, the ratio of fundamental wave-length to oscillator length increases from 2.7 to 4.8, making progress increasingly difficult. To measure the radiation, various improved radiometer receivers were developed. Receiving elements either of short lengths of 1 μ platinum wire or of narrow strips of platinum film deposited on thin mica were substituted for the usual black vanes. The sensitive systems were suspended by quartz fibers and weighed only ½ to 1 mg. By proper choice and adjustment of the elements, the receiver can be made either selective or not. By direct test, the deflection was proved proportional to the incident energy. Mirrors and paraffin lenses were used in focussing the energy from the oscillator on the receiving element. Wave-length measurements were made with the aid of a Boltzmann divided mirror interferometer, and energy-distribution curves were obtained for various oscillators and receivers. To correct for the variations in the emission of the oscillator, readings were made, simultaneously, of the total emission with a check receiver, and of the radiation of a particular wave-length. On account of the high damping of the oscillator, the curves obtained show the predominant influence of the fundamental frequency of the receiver but also maxima when partials of the oscillator coincide with one or other of the partials of the receiver. The shortest fundamental wave-length obtained is 1.8 mm, two octaves shorter than the shortest previously measured, 7 mm. An upper partial of 0.8 mm was also observed. Greater homogeneity was secured by use of a reflecting echelon, $\frac{1}{2} \lambda$ between treads, since it reflects selectively. A simple mounting permitting easy adjustment is described. Two such echelons with steps at right angles give, under suitable conditions, a fair approximation to monochromatic radiation. By use of such echelons it should be possible to isolate upper partials still shorter than those so far measured.