ON A NEW (?) METHOD IN EXTERIOR BALLISTICS.^ 



By a. G. WEBSTER and MILDRED ALLEN. 

 (Read April 20, 1918. Received June 4, 19 19.) 



Whether the method to be described deserves to be called new 

 or not is a matter of opinion, since the instrument here used was 

 designed and built by one of us twenty-eight years ago, and has 

 been described in an article in the Physical Review, Volume 6, May- 

 June, 1898. The method of measuring time by means of the charg- 

 ing or discharging of a condenser has been used by Pouillet, and has 

 been applied to ballistics by Cranz in Germany and Sabine in Eng- 

 land, but so far as we know the electrometer has not been applied 

 to ballistics. 



The essential part of the instrument for measuring the time is 

 shown in Fig. i. A projectile drops from an electromagnet upon 

 the two levers shown in the middle, the one on the right being car- 

 ried on a carriage adjustable by means of a micrometer screw. By 

 knowing the height through which the projectile falls, the velocity 

 of the projectile on striking the lever is known. 



The arrangement of the apparatus is shown in Fig. 2. The 

 battery charges the condenser through a resistance, the condenser 

 being short-circuited through conductor {a, b), which in the calibra- 

 tion is the left hand lever of the drop interrupter. When the circuit 

 is broken the condenser begins to charge in accordance with the 



equation 



_ t 

 q = qo(i - e -^'^). 



A curve of calibration is shown in Fig. 3, by which it appears 

 that times may be measured of the order of one-millionth of a 

 second. 



In the ballistic application (a, b) and (c, d) are two strips of tin 

 foil stretched tightly between the brass supports. These are shot 



1 Contribution from the Ballistic Institute, Clark University, No. 3. 



382 



