﻿336 Prof. R. BalPs Lecture Experiments to 



pendulum ; so, for the sake of making this with the necessary 

 delicacy, the screw X has been introduced into the contact- 

 breaker, by raising or lowering which the limiting position of 

 the spring and therefore of the saddle is raised or lowered. 

 By this means the amount by which the pendulum depresses 

 the spring in its transit over it can be arranged with the great- 

 est nicety. After the pendulum has received an impulse, and at 

 the bottom of its swing comes into contact with the saddle, the 

 current is broken owing to the depression of the spring from X. 

 After the point has passed the saddle the spring returns to its 

 bearing, and the current flows again until the return of the 

 pendulum to the lowest point, when the current is broken again ; 

 and if the contact-breaker have been nicely adjusted, exactly a 

 second will have elapsed between these two breakings. Thus at 

 the completion of each second the current is interrupted. The 

 apparatus being nicely adjusted, the amount of vis viva lost by 

 the pendulum in depressing the spring is so small that it will 

 make, after having received an impulse, upwards of two hun- 

 dred breakings before it requires another push. 



An electromagnet acting on a bell is introduced into the cir- 

 cuit, so arranged that at each interruption of the current the bell 

 is struck. The simple arrangement necessary for this need not 

 be described. At every oscillation of the pendulum the bell 

 rings, the sounds of which may therefore be regarded as the tics 

 of the pendulum rendered easily audible to the whole room. 



The reason why the spring or middle of the saddle of the con- 

 tact-breaker should be placed exactly under the point of suspen- 

 sion of the pendulum can now be easily seen. It is in order 

 that the intervals of two consecutive strokes of the bell shall be 

 exactly equal. If the spring have not the correct position, then 

 there will be two intervals, one as much greater as the other is 

 less than a second, and these intervals will alternate. * Errors 

 which arise from the want of perfect adjustment of this position 

 are fortunately rendered insensible by the fact that at its lowest 

 point the pendulum has its maximum velocity. 



It will be noticed that with this contact-breaker the interval 

 between the strokes is independent of the arc of vibration. 



I am not aware that this mode of breaking contact has been 

 used before, and therefore I have given the details. It ap- 

 pears convenient, efficient, accurate, and not liable to derange- 

 ment, and is free from the troubles (in my experienee unavoidable) 

 arising from using mercury or other fluids for the same purpose. 



The mode of proving that a body falls sixteen feet in a second 

 is then easily seen. A large scale divided into feet is attached 

 to the upright support of the pulley. One or both of the elec- 

 tromagnets belonging to the triangle, the contact-breaker, and 



