THE TEACHING OF MECHANICS BY EXPERIMENT. 257 
spiral spring the suspension being carried through a graduated steel 
rule which allows the amount by which the spring extends to be read 
by a telescope. This is conveniently made self-contained : the telescope 
is attached to the same stand as carries the loaded spring. The student 
verifies Hooke’s law and determines how much extension is produced 
by a given load ; he then sets the weight vibrating and observes the 
record of vibration and from these two observations taken together 
he can work out the value of “ g.” 
Another simple dynamical experiment of great value is the determin¬ 
ation of the period of oscillation of a simple pendulum and the com¬ 
parison of it with the period of another pendulum of the Kater type. 
The simple pendulum consists of a small ball hung by a thread. The 
other is a solid rod suspended on a knife-edge and capable of being 
suspended on another knife-edge near its other end, and carrying 
a weight clamped on it The weight on it is divided into two parts 
whose positions are separately adjustable by means of screws. The 
student adjusts them until the period about one knife-edge is the 
same as the period about the other knife-edge, and he then observes 
that the distance between the knife-edges is equal to the length of 
the simple pendulum when that is regulated so as to vibrate in the 
same period. 
This apparatus shown in figure 4 is a species of compound ballistic 
pendulum or balance. It consists of a heavy horizontal beam 
Fig. 4. Compound Ballistic Balance. 
