someivhat analogous to Newton s Rings. 83 



nearer together. The wires in question were O55 cm. diameter, and, 

 when placed at 66 cm. apart, there was practically no reflexion. 



COMPARISON OF THEORY AND EXPERIMENT. 



It now becomes of interest to compare the results of Experiments 

 V to IX with the theories advanced concerning them. 



In Experiment V we have the capacity per unit length of the 

 normal part in electrostatic units approximately given by 

 I/ (4 log e djw), where d is the distance between the centres of the 

 wires and w their radius. For the abnormal part, without any cor- 

 rection for the edges, we have capacity per unit length equals D/4?r(#, 

 where D is the width of the tinfoil sheets. Now d was 8 cm., w was 

 0-05 cm. nearly, and D 32 cm. Thus we obtain for r, the ratio of the 

 capacities, 6 P 5 nearly. If, however, in estimating the capacity of the 

 abnormal part, a correction for the edges is made, we obtain values 

 of r ranging up to 8 or 9, according to the length of the abnormal 

 part under consideration, and the formula used for the correction. 



Taking as a typical case the length of abnormal part to be 

 2'25 m., on which special experiments were made, and correcting for 

 the edges by the approximate formula given by Professor Kohlrausch* 

 we obtain r = 9, nearly. Whence, from equation (1), It 0*8, 



It will be seen from the equations (3) that it is further necessary to 

 know the constants a and /3. These were determined by experiments 

 similar to those previously devised and carried out by V. Bjerknes.f 



In my case, however, the electrometer readings were alternated 

 with the bridge at the variable distance a?, and at the quarter wave- 

 length distance beyond the electrometer. A spark gap of 2 mm. was 

 used throughout. 



Thus were obtained for the electrical waves advancing along the 

 wires, the wave-length X x = 9m.; and for the constant involving the 

 primary damping, we have 7! = 2^/3 = 0*524, or say 7 Z = 0'5 nearly. 



Then, assuming that the velocity of propagation of the waves along 

 the wires is practically that of light in air,J we obtain, from 



* ' Leitfaden der praktischen Physik,' Leipzig, 1892, p. 357. 

 f Wiedemanu's * Annalen,' yol. 44, pp. 519522, 1891. 



This is known to be the case from various experiments, and also from theo- 

 retical considerations. See, for example : 



(1.) Professor Hertz, " Ausbreitung der elektrischen Kraft," Leipzig, 1892, or 



the English translation by Professor D. E. Jones, B.Sc. 

 (2.) MM. Sarasin and De la Eire, 'Archives des Sciences Physiques et 



Naturelles,' vol. 29, No, 5. Geneve. 1893. 

 (3.) Professor J. J. Thomson, F.E.S., "Kecent Eesearches on Electricity and 



Magnetism," pp. 279 and 451 467. 

 <4.) Professor Oliver Lodge, F.E.S., ' Phil. Mag.,' August 1888, vol. 26, p. 228 ; 



' Eeport Brit. Assoc.,' 1888, p. 567 ; and f Proc. Eoy. Soc.,' vol. 50, pp . 



2939. 



