292 



Professor A. M. Worthington 



[May 18, 



of the mercury a bright self-induction spark in the neighbourhood 

 of the splash, and it is by this flash that the splash is viewed. The 

 illumination is greatly helped by surrounding the place where the 

 splash and flash are produced by a white cardboard enclosure, seen 

 in Fig. 2, from whose walls the light is diffused. 



It will be observed that the time at which the spark is made will 

 depend on the distance that the sphere has to fall before striking the 

 plate D, for the subsequent action of demagnetising F and pulling 



Fig. 2. 



the wire L out of the mercury in the cup H is the same on each 

 occasion. The modus operandi is consequently as follows: — The 

 observer, sitting in comparative but by no means complete darkness, 

 faces the apparatus as it appears in Fig. 2, presses down the ends 

 A' B' of the levers first described, so that they are held by the electro- 

 magnet C (Fig. 1). Then he presses the lever NP down on the 

 electromagnet F, sets the timing sphere and drop in place, and then 

 by means of a bridge between two mercury cups, short-circuits and 



