218 



Messrs. Trowbridge and Duane on 



plate. The total length of the secondary circuit from one 

 condenser plate through E, G, J, H, F to the other plate was 

 6338 centim. The circuit consisted of copper wire (diameter 

 '215 centim.) supported at each end by suitable wooden frames, 

 and also once in the middle by hard-rubber hooks, fastened by 

 long pieces of twine to a wooden crossbar above. The dis- 

 tances from F to E and from K to L were 30 centim., and a 

 spark-gap with pointed tin terminals was inserted at J. The 

 primary circuit consisted of copper wire (diameter *34 centim.). 

 The distances between the two parts AB and CD were 

 45 centim. The portion B D contained a spark-gap with 

 platinum-faced spherical terminals, and was made so as to 

 slide back and forth, to and from the condenser. The motion 

 of this movable piece varied the self-induction, and therefore 

 the period of oscillation of the primary circuit. By this 









Fig. 3. 

 E G 



K 





D 



r> 



a ~ 





B 

 A 





A — 



r 





















c 

 ft 









g 







J , 



f 









J 









y 



I . 





1 



h J 







F H 



L 





M 



M 



means the circuits were brought into resonance. With certain 

 arrangements of the condensers the resonance was very 

 sharp, and the position of the movable portion could be deter- 

 mined to within *25 centim. In the arrangement which was 

 finally adopted the resonance was not so sharp. Even in 

 this case the distance of the sliding part from the plate a 

 could not have been in error by more than 2 centim. The 

 length 65 centim. was finally chosen for its value. 



The automatic current-interrupter that worked so beauti- 

 fully in connexion with the Hertz vibrator would not function 

 well when used to excite the circuits just described. After 

 trying many devices, we finally adopted an ordinary reed 

 interrupter with a comparatively large hammer-and-anvil 

 arrangement, which gave little trouble. 



At first it was found impossible to produce anything but a 

 complex vibration in the secondary circuit when the spark- 

 gap was open. Some slight evidence of resonance was 

 obtained, but nothing of a decided character. When, how- 

 ever, the spark-gap was closed, very good resonance ensued, 

 and a wave the length of which could be measured to within 

 *4 per cent, was excited. Some photographs were taken of 



