324 Messrs. Trowbridge and Sabine on 



The researches of Feddersen upon electrical oscillations* 

 were more quantitative than those of Joseph Henry : and 

 Lorenz f , by his repetition of Feddersen's results, and by his 

 mathematical analysis of them, apparently gave subsequent 

 observers a solid basis for calculation. 



The results of Feddersen and of Lorenz were obtained by 

 photography. An image of the electric spark drawn out by 

 means of a revolving mirror was photographed, and the dis- 

 tances between the successive oscillations, shown by dark 

 bands on the photograph, were measured. Lorenz assumed 

 the ratio between the electrostatic units and the electromag- 

 netic units, ^ = 300 x 10 6 metre, as that of the velocity of 



light ; and by means of the formula t= — v / LC obtained a 



satisfactory agreement between the result of experiment and 

 the theory. He showed, apparently, that a certain lack of 

 agreement between theory and experiment, which Feddersen 

 had noticed, was due to taking the dielectric constant of glass 

 too small. 



It will be noticed that the method of Feddersen, by means 

 of which the electrical oscillations are photographed, appa- 

 rently affords an accurate method of determining v. For the 

 factors L and C occur under the square root, and the per- 

 centage errors of determination of L and C, being under the 

 square, are halved. Lorenz did not repeat the entire work of 

 Feddersen, but only obtained a sufficient number of photo- 

 graphs — taken under definite conditions in regard to capacity 

 and inductance of the circuit — in order to measure t, the time 

 of oscillation. The accuracy of the results which can be 

 obtained for v depends upon the limits of accuracy of the 

 measurements of the photographs, and of the determinations 

 of the dielectric capacity for oscillatory charges. 



In reasoning upon the mode of electrical oscillations in 

 dielectrics, it occurred to us that the medium of the dielectric 

 must greatly influence the result. At the instant the elec- 

 trical oscillations occur, the glass of the Leyden jar is subjected 

 to a strain which is more or less periodic. It is not probable 

 that the capacity of a condenser is the same for rapid charges 

 and discharges as for slow ones, and the measurements of 

 capacity by the ordinary slow methods form no criterion of 

 the capacity of glass under electrical influences which last but 

 three millionths of a second. We therefore concluded to 



* Poggendorff's Annalen, vol. ciii. p. 69 (1858) ; vol. cviii. p. 497 

 (1859); vol. cxii. p. 452 (1861); vol. cxiii. p. 437 (1861); vol. cxv. 

 p. 336 (1862); vol. cxvi. p. 132 (1862). 



f Wiedemann's Annalen, vol. vii. p. 161 (1879). 



