110 PROCEEDINGS OF THE AMERICAN ACADEMY 



of them, apparently gave subsequent observers a solid basis for 

 calculation. 



The results of Feddersen and of Lorenz were obtained by photogra- 

 phy. An image of the electric spark drawn out by means of a revolv- 

 ing mirror was photographed, and the distances between the successive 

 oscillations, shown by dark bands on the photograph, were measured. 

 Lorenz assumed the ratio between the electrostatic units and the elec- 

 tromagnetic units, V = 300 X 10*^ m., as that of the velocity of light; 



and by means of the formula < = — y^/, (J 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, apparently affords 

 an accurate method of determining v. For the factors L and C occur 

 under the square root, and the percentage errors of determination of 

 L and G, being under the square, are halved. Lorenz did not repeat 

 the entire work of Feddersen, but only obtained a sufficient number 

 of photographs — 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 pho- 

 tographs, 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 in- 

 fluence the result. At the instant the electrical 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 meas- 

 urements 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 employ an air con- 

 denser instead of one of glass, in order to detect, if possible, the effect 

 of the medium of the dielectric upon electrical oscillations. In order 

 to obtain sufficient capacity for a suitable spark, we were obliged to 

 use the cylindrical form of condenser. The first condenser we em- 

 ployed was made of sheet zinc, and consisted of nineteen coaxal cylin- 

 ders. The inner cylinder had a diameter of 15.1 cm., and the outer, 



