256 Prof. C. V. Boys on PhotograpJis of Rapidly Moving 



denser will safely stand. A string tied to the prop is then 

 pulled, and the spring completes the second gap so quickly 

 that not more than one or two oscillations can take place after 

 the spark hegins and before the filling of the air-gap at the 

 trap. Thus a higher potential can be obtained than would be 

 possible if the short working spark-gap alone determined the 

 potential, while the resistance in circuit is less than before 

 owing to the shortness of the spark. 



Two sparks taken with the trap are shown in B, fig. 6. 

 Two longer sparks taken without a trap are reproduced in 

 C, fig. 6. The gain in number of oscillations is not very great, 

 as the measures by Messrs. Kobson and Smith at the end of 

 this paper show ; but there is a gain, especially as compared 

 with a short spark taken without a trap. The trap, however, 

 is practically useful in another way, it enables one to fix 

 the instant at which the spark shall occur, which, besides 

 being convenient, is important if for any reason the highest 

 possible speed of rotation is required. While the lenses will 

 run continuously at so low a speed comparatively as fifty or 

 seventy revolutions a second, it would not be well to run them 

 at high speeds for more than a few seconds together. By the 

 use of the trap this is possible, whereas, if the high speed had 

 to be kept up during the long time that it takes to charge a 

 condenser of so great a capacity to the spar king-point, there 

 would be great risk to the bearings of the wheel of lenses. 

 For instance, to obtain a spark about one tenth of an inch 

 long from the condenser used in these experiments, from 

 twenty to thirty turns of the handle of a large eight-plate 

 Wimshurst machine had to be made. 



Anomalous variations of brilliancy are not unusual, as may 

 be seen in A, but a curious example is indicated by the letter 

 p in D. This represents the latter or dying portion of a 

 spark taken on the same negative and under the same general 

 conditions as A. In this case it will be seen that the light 

 instead of going out at the end of an oscillation travelled 

 slowly across the gap to the other terminal, forming apparently 

 a kind of ball-discharge. 



One series of photographs I may here refer to, though 

 the effect sought for is not so evident as I had expected. In 

 this series a short vertical spark was formed, with its length 

 parallel to the direction of the motion of the lens which at 

 any time might be opposite to it. A magnetic field was pro- 

 duced in the line joining the spark and the lens. Then it was 

 hoped that the current oscillating backwards and forwards 

 would be largely deflected laterally alternately on either side 

 so as to produce a zigzag impression. The arrangements 



