﻿Resonance and Electrical Interference. 185 



employed for the primary circuit consisted of two large iron 

 frames, upon which sheets of tin were screwed. The con- 

 denser-plates thus made were 210 centim. hy 330 centim., and 

 were placed 2 centim. apart. The air-condenser employed in 

 the secondary circuit replacing at times the hard-rubber con- 

 denser D is described, together with the photographic appa- 

 ratus, in a previous paper on Electrical Oscillations *. It was 

 placed in a room provided with yellow window-shades (orange 

 fabric). It was found necessary also to construct a camera- 

 box about 10 feet long, which extended from the spark-gap to 

 the revolving mirror. This box was so constructed that the 

 direct light of the sparks was shielded from the sensitive plate 

 which was placed directly below the spark-gap. The plate 

 thus received only the light thrown by the revolving mirror, 

 which was placed at the opposite end of the camera-box. 

 The operator, seated at the spark-end of this camera, closed 

 for an instant a key K (woodcut, fig. 1) placed in the circuit 

 of the alternator M. Looking through the film of the sensitive 

 plate, one could determine when a suitable photograph had 

 been taken ; for the image of the sparks spread out by the 

 revolving mirror could be clearly seen, the film acting like a 

 ground-glass. It was thus possible to take a large number of 

 photographs with the greatest ease, the rate of the alternator 

 being so high that at each sudden make of the key several 

 photographs could often be obtained at once upon the strips 

 of sensitive plates, which were 25 centim. by 6 centim. It 

 is evident that one of the circuits, for instance B, could be 

 made a time-circuit. The self-induction and capacity in the 

 circuit could be carefully determined and maintained con- 

 stant. The formula t = 27r \/~L C could be thus printed, so to 

 speak, on each negative. For this time-circuit I have 

 employed a fine wire coil which was slipped upon the same 

 electromagnet which formed the primary of the step-up 

 transformer T. I had thus two step-up transformers with a 

 common primary : one produced the sparks of the vibrator, 

 the other the spark of the time-circuit. 



In fig. 2 (Plate VII.) S' represents photographs of the 

 unidirectional primary spark. S is the unidirectional spark 

 produced in a neighbouring circuit B from which the capa- 

 city has been removed. S" is the oscillating spark in the 

 circuit C : the condenser of this circuit was an air-con- 

 denser. The spark S shows that no oscillation is concealed 

 by the heavy pilot-spark of the exciting spark S'. The 

 photographs S" show that the unidirectional spark S' can 



* Proc. Amer. Acad. Arts and Sciences, May 28, 1890 ; Phil. Mag. [5] 

 xxx. p. 323 (October 1890), 



