﻿338 Prof. R. BalFs Lecture Experiments to 



likewise covered with a piece of tinfoil, which pieces, however^ 

 must at no place communicate with one another. Each edge is 



Fig. 3. 



furnished with a binding-screw, to which a wire is attached. 

 Whenever the ball rests on the groove, the tinfoil enclosing it 

 touching each edge completes the electric connexion between 

 the two binding-screws, the ball acting as a bridge along which 

 the current passes. At D, and the similar point on the other 

 side, the two ends of a piece of india-rubber spring are fastened 

 so that the ball can be grasped by the spring. When the ball 

 is pulled up along the groove and then released, the force of the 

 spring pulls it down and it darts off with a' horizontal velocity. 

 This piece of apparatus may be about seven feet from the ground. 

 At precisely the same vertical height as C, and at a distance of 

 some feet, an electromagnet, M, is to be supported. One wire 

 from this goes to the battery, the other is fastened to one of the 

 binding-screws on one edge of A C, the second edge being con- 

 nected with the other pole of the battery. 



As long as the tinned ball P is on the groove, the circuit being 

 complete, the electromagnet, M, will sustain a second ball It ; but 

 the moment P leaves the groove, It is released. Drawing back P 

 and the spring which embraces it and then releasing it, P main- 

 tains the circuit until it arrives at C ; in fact a good contact is 

 ensured by the double circumstance that both the spring and the 

 centrifugal force of P conspire to keep it in close contact with 

 the tinned edges. After leaving C the ball darts off in the tra- 

 jectory indicated in the figure; but directly it is free It is like- 

 wise free, and the two can be seen with the greatest exactness to 

 reach the ground together. 



By stretching the spring more or less, any amount (within 

 reasonable limits) of horizontal velocity can be communicated to 

 P; and it is a most striking result to observe in all cases the 

 perfect simultaneity with which the two balls reach the ground. 



