Vol. 6, 1920 
PHYSICS: THOMPSON, HICKMAN, RIFFOLT 
Another application of the method of time caHbration and of determin- 
ing velocities comes in an entirely different field, namely, the measure- 
ment of the velocity of light. With a range of about 40 or 50 miles, 
which is available, a simple apparatus of this type will provide a record 
comparing the instant of arrival of light directly from the source (one of 
short duration, such as a spark or very rapidly moving slits) and that which 
has gone out and back over the 50 miles. The record will be similar to 
those which have been shown and obtained in much the same way. A 
precision which is at least as great as that obtained by the other methods 
which have been used for the measurement of the velocity of light is to 
be expected and in addition the permanent record, which will be given by 
a deflection on the film of the order of two centimeters, this representing 
the time taken for the light to travel the 50 miles. 
The shadowgraphs (fig. 5) of projectiles in flight were made by use 
of the apparatus shown in figure 6, a diagram of which is shown in figure 
7. The spark is obtained by breaking a small wire {w) through which the 
current passes to the primary of a large coil. The current is applied just 
before firing the gun, and in this way a very small wire can be used, not 
having time to fuse. The secondary condenser C controls the time inter- 
val elapsing between breaking the wire and the passing of the spark through 
the gap G which is constructed as shown in the figure. This makes a good 
"point" source and one always in the same place. The lens L is quartz. 
The position of the projectile on the plate can be controlled to perhaps 
1 centimeter by moving the wire {w) with respect to the plate. This 
corresponds to a time interval of V 75, 000 second. 
FIG. 7 
