the Velocity of Electric Waves. 217 



circuit depend to a large extent upon the character of the 

 primary spark, and that the most active sparks are those 

 between metallic spheres with polished surfaces. It is true 

 that waves can be produced by sparks between points, but the 

 oscillations are not so powerful or well marked. In the 

 second place, from the results obtained by Bjerknes one 

 would expect the oscillations in the secondary circuit to be 

 much less damped than those in the primary. This expecta- 

 tion has been fully realized. Photographs show from ten to 

 twelve times as many oscillations in the secondary as in the 

 primary. The longest secondary spark we counted indicated 

 60 complete oscillations. In the third, and by no means the 

 least important case, the question how close the resonance is 

 does not affect the accuracy of the results. By photographing 

 the sparks in the secondary the period of oscillation is 

 determined, not of a circuit that is altered until by trial it is 

 found to have as nearly as possible the same period of vibration 

 as the circuit on which the length of the wave is measured, 

 but that of the circuit along which the wave itself is actually 

 travelling ; and hence the conclusions in regard to the effect 

 of damping reached by Bjerknes in his admirable paper on 

 "Electric Resonance"* do not affect the accuracy of the results. 



The great difficulty to be overcome is the production of 

 secondary oscillations that will produce sparks sufficiently 

 bright to photograph. It is comparatively an easy task to 

 photograph the primary spark, but in order to photograph 

 the secondary the dimensions of the circuit must be chosen 

 with great care. 



With a view to increasing the light of the spark, together 

 with the length of the waves, it seemed desirable to lengthen 

 the period of oscillation by enlarging the condensers rather 

 than by increasing the self-induction of the primary circuit. 

 A castor-oil condenser therefore was designed and constructed 

 on the following plan : — Eight plates (25 centim. x 20 centim.) 

 were cut out of sheet zinc, and were held in vertical planes 

 side by side 2 centim. apart by a suitable hard-rubber frame. 

 The plates were entirely immersed in castor-oil contained in 

 a glass jar. They were connected together in the manner 

 shown in fig. 3. The plates marked a, c, and e were fastened 

 to the conductor AB, and formed one armature of the con- 

 denser. Those marked d, /, and h were joined to C D and 

 formed the other armature. The two ends of the secondary 

 circuit E, G, J, H, F were fastened to the plates b and g. The 

 plane of the secondary circuit was 50 centim. and that of the 

 primary 3 centim. above the upper edge of the condenser 



* " Ueber electrische Resonanz," Wied. Ann. lv. p. 121 (1895). 

 Phil. Mag. S. 5. Vol. 40. No. 243. Aug. 1895. Q 



