56 Chant — Variation of Potential along a 



the same as those of the oscillator; in other words, the oscilla- 

 tions were not forced along the wires, but were those natural 

 to them. 



These experiments were verified by Rutherford,* who sub- 

 stituted a magnetic detector for the bolometer, and found it 

 equally sensitive. 



In all these experiments it will be observed that the wave- 

 lengths determined along the wires are not those proper to the 

 oscillator, but are either due to the detector used or to the 

 wires vibrating naturally. 



In Birkeland'sf and Jones'sJ researches the reverse seemed 

 to be the case. The wire-systems were both similar to that of 

 Sarasin and de la Rive, but the means of exploration were 

 different. Birkeland examined the potential at various points 

 of his wire, which was 30 m long, by measuring the length of 

 the spark which leaped from it to an earth connection, the 

 existence of the spark being indicated by a telephone receiver 

 held to the ear. He obtained minimum points which varied 

 as the period of the oscillator was changed, thus indicating 

 that the measured wave-length depended on its period. These 

 minima were unequally spaced, which irregularity was attrib- 

 uted to the damping of the waves and the loss on reflection at 

 the ends of the wires ; but the explanation does not seem 

 entirely satisfactory.§ 



Jones used a thermal junction inserted at different points of 

 his wire, which was 130 m in length, the effect being indicated 

 by a low-resistance galvanometer in circuit with it. The oscil- 

 lator was of the usual type, with plates 40 cms square, but the 

 length of the connection between the plates is not given. Sev- 

 eral well-defined maxima and minima were observed, and the 

 wave-length was determined to be approximately 4*3 m . It is 

 interesting to read that " several curious results were recorded 

 for which no explanations were forthcoming." || 



It may be worth while to recall how the wave length of 

 Hertz's plate oscillator was determined. He obtained it with 

 the aid of his circular resonator, but, as has been already 

 remarked, the wave-length thus found is always eight times the 

 diameter of the resonator. If, then, we could know when the 

 resonator was exactly in unison with the oscillator, the wave- 

 length could be deduced with considerable accuracy. But this 

 is not at all possible; the resonance is far from being sharply 

 defined. Indeed, Hertz says that the same resonator, of diam- 

 eter 70 cms , was in resonance with three different oscillators. 

 The first consisted of two spheres of diameter 30 cms , connected 



*E. Rutherford, Phil. Trans. A, 1897, clxxxi, p. 1. 

 |Kr. Birkeland, Wied. Ann., xlvii, p. 583,1892. . 

 I'D. E. Jones, Brit. Assoc. Report, 1891, pp. 561-2. 



\e Poincare, 1. c, p. 176. || Nature, xliv, p. 454, 1891. 



