the Velocity of Electric Waves. 221 



Outside the plates a and b, and separated from them by a 

 hard-rubber dielectric (K = 2 + about) 1'8 centim. thick, 

 were the secondary plates, 26x26 centim. The primary and 

 secondary circuits were joined to the condenser plates as 

 indicated in the figure. The primary circuit lay in the hori- 

 zontal plane passing through the centres of condenser plates, 

 and consisted of copper wires '34 centim. in diameter. In order 

 to control the period of oscillation of the primary circuit, the 

 portion B D containing a spark-gap with spherical terminals 

 was made, as before, so as to slide along parallel to itself. 

 The distance between the straight portions A B and C D was 

 40 centim., and the lengths of A B and C D finally chosen 

 for best resonance were 85 centim. Most of the secondary 

 circuit lay in a horizontal plane 16 centim. above that of the 

 primary. The lengths G E and H F, however, were bent 

 down and fastened to the middle points G and H of the 

 secondary plates. The circuit consisted of copper wire (dia- 

 meter *215 centim.), and its total length from G through J 

 to H was 5860 centim. At J was a spark-gap with pointed 

 terminals. With this apparatus we succeeded in producing a 

 very regular wave-formation, as indicated by the bolometer, 

 even when there was a spark-gap at J. So many curves 

 have been plotted and published to illustrate the charac- 

 teristics of electrical waves that it does not seem worth while 

 to add to the number here. It will be sufficient to state that 

 the ratio of the maximum and minimum deflexions in the 

 bolometer was about 15 : 1, and that there was a node at J, 

 and another about 40 centim. to the right of E and F. 



Upon photographing the secondary spark some curious 

 phenomena were observed. In the first place the dots usually 

 appeared in pairs. There would be two black dots followed 

 by a space where two or three dots either appeared faintly or 

 were absent altogether, after that the two black dots would 

 reappear followed again by a faint space, and so on for six or 

 seven repetitions. All this of course occurred in a single spark. 



The explanation that first presents itself is that the two 

 black dots are the result of the first two oscillations in the 

 primary circuit, which, owing to the damping, are much more 

 powerful than the others. If this were the true reason the 

 first of the pair of dots always ought to be blacker than the 

 second, and every third dot ougdit to be the first of a pair. 

 This is not the case, however. On the other hand, the phe- 

 nomena cannot be explained as the result of a complex 

 vibration ; for the bolometer-readings, taken only a few 

 minutes before the photographic plates were exposed and 

 with exactly the same arrangement of apparatus, indicated ex- 

 tremely regular waves. A clue to the mystery was furnished 



