﻿illustrate the Laws of Motion. 



335 



The next experiment is to prove that a body falls sixteen feet 

 in the first second. The apparatus already described is em- 

 ployed for this purpose, but into the same circuit two other parts 

 are introduced which will require a few words of explanation. 



The contrivance on which the arrangement principally depends 

 is the contact-breaker, the action of which will be understood 

 from the annexed diagram (fig. 1). To a block of wood, AB, a 



Ffc. 1. 



brass pillar P is screwed. This pillar is 3 inches high, and has 

 a binding-screw attached to it to receive the current. Near the 

 top of the pillar a very slender spring is rivetted ; this spring is 

 of brass wire slightly flattened, and is 8 inches long ; at the point 

 T, five inches from P, it bears what may be called a saddle. 

 This consists of a piece of ordinary tin plate cut into a rectangle 

 of 1 inch by J an inch, soldered lengthways on the upper sur- 

 face of the spring and then bent down on each side, so that its 

 section is similar to what is represented in fig. 2. The object of 

 this will presently appear. The other end of the Fig. 2* 



spring is free, but it bears against a screw, X, ^*r*«^ 

 which turns in a brass piece, Q, likewise screwed x^-^^\ 

 to the block of wood AB. The spring being 

 weak enough, the slightest touch will depress the end of it from 

 X, to which, however, it immediately returns on the relaxation 

 of the pressure. When the spring (along which the current tra- 

 vels) touches X the circuit is complete, and it is of course inter- 

 rupted when the spring is depressed. 



A seconds-pendulum is suspended from a suitable portion of 

 the framework by a spring in the usual manner. No clock- 

 escapement is used ; indeed, if the bob be heavy, the pendulum 

 once set in motion will vibrate for some minutes without requi- 

 ring an additional impulse. Underneath the pendulum the con- 

 tact-breaker is to be placed in such a manner that its spring is 

 normal to the plane of vibration of the pendulum ; and the height 

 of the pendulum must be so adjusted that when the bob is in its 

 lowest position, a point attached to it shall just touch the saddle 

 in passing over it. The final adjustment, however, is incon- 

 venient to make by moving the point of suspension of the heavy 



