Oscillatory Discharges. 3 



discharges, numerous researches have been performed, either 

 to show experimentally the existence of electric oscillations, 

 or to test the well-known theoretical formula 



a) 



/ 1_ _R*' 



V LO 4L 2 



established by W. Thomson [Lord Kelvin] for the period of 

 oscillation. 



These experiments — which we think useful to record briefly 

 — may be divided into two groups, according as they were 

 made in order to ascertain the value of T by Feddersen's 

 method, or with a view to determine the curve representing 

 the time variations of the intensity of the charge or discharge 

 current of a condenser. 



4. Experiments performed with Spark-Photographs. — 

 H. Feddersen * was the first to measure the period of the 

 oscillatory discharge of a condenser, by photographing the 

 spark after its reflexion by a rotating mirror; but as he 

 did not make any absolute measurements of the capacity and 

 self-induction, he only obtained qualitative laws, which we do 

 not think worth while recording- here. 



Feddersen's experiments were then repeated by Lorenzf, 

 who stated that the values calculated for the duration of 

 oscillation of the discharge, though corresponding to those 

 observed as regards order of magnitude, were always somewhat 

 higher, probably on account of the too small value ascribed 

 to the dielectrical constant of the glass forming the insulating 

 medium of the condensers used. 



In order to eliminate this cause of uncertainty, Trowbridge 

 and Sabine % tested the discharge of an air-condenser by pho- 

 tographing the spark by means of Fedderseir's method. They 

 found the formula 



T = 27rVT(T, 



to which (1) is reduced for small values of R — as was the case 

 in their experiments — to be fairly well verified, provided 

 the value of L as calculated by Rayleigh's formula for 

 rapidly alternating currents be adopted. The values of T 

 obtained are of the order of 0*0000031 sec, in most satis- 

 factory agreement with theoretical values. 



* Pogg. Am. ciii. p. 69 (1858) ; cviii. p. 497 (1859) ' cxiii. p. 437 (1 SO 1 ) ; 

 cxvi. p. 132 (1862). 



t Wied. Ann. vii. p. 161 (1879). 

 % Phil. Mag. xxx. p. 323 (1890). 



B 2 



