186 
IOWA ACADEMY OF SCIENCE 
28.2 
28.7 
28.4 
/ 28.8 
28.9 
av. 28.6 scale divisions. 
The time required for the hall to fall 6.2 cm is 0.1125 seconds. Thus the 
value of K, i. e. the time required for one division deflection, is 0.00393 sec. 
By discharging a 0.1 micro farad condenser, after charging to the same 
potential, viz., 19.2 volts, the deflections were: 
109. 
108.5 
109.0 
109.1 
109. 
108.9 
109.3 
109.0 
108.7 
108.6 
av. 108.7 scale div. 
By the calculation with the aid of equation 1, the value of K is 0.00387 sec. 
per scale division. 
SOME ADAPTATIONS OF THE APPAKATIJS. 
The apparatus can be adapted wherever time can be marked by making and 
breaking electrical contacts. One illustration is in the verification of the laws 
of falling bodies. It is well known that the laws of gravity can not be verified 
directly for bodies falling any large distance because the friction of the air 
becomes an appreciable factor. But for short distances the velocity is so small 
that the air friction can be neglected. The photograph in Pig. 2 shows the 
apparatus as used to verify the laws directly for a ball falling a distance vary- 
ing from 1 mm to 400 mm. The moving part of the keys shown were made of 
light aluminum, so that a very short time would be required for them to get 
in action. The upper key is held up by a thread which runs over a smooth 
rod, and also supports the ball. The ball and the upper key are released 
simultaneously by burning the thread. The lower key is thrown by the impact 
of the ball. The resistances, a, b, R, x and Ax are all included in the one small 
box behind the ball. 
A sample of some observations are given in the curve in Pig. 3. The time as 
plotted was calculated from the formula s=l /2 gt 2 . Three readings were aver- 
aged for each indicated point on the curve. The deflection is here shown to 
vary directly as the time. 
A second application of the method is for the measurement of the velocity 
of rifle bullets. The great difficulty in this adaptation was in getting keys of 
such small inertia that they ' would close and open the electrical circuits by 
the passing of the bullet. A bullet will pass through a screen of paper or thin 
