A PHOTOELECTRIC CELL METHOD OF MEASURING THE VELOCITY OF PROJECTILES

Abstract

The velocities of rifle bullets have been measured by causing the bullet to interrupt successively beams of light shining on photoelectric cells. The resulting current pulse in the photoelectric cells, after being amplified, actuates a recording device. Two methods of taking the records were used. In the first the amplified current from the photoelectric cells is recorded by an oscillograph consisting of an Einthoven galvanometer and a rotating drum camera. In the second method each current pulse from the amplifier caused by the interruption of the beam of light by the bullet actuates a thyratron so that it discharges a condenser. The current through the condenser is recorded by the Einthoven galvanometer. The time record for both methods is obtained from a valve-maintained tuning fork having a frequency of 1000 cycles per second. The output of the tuning fork circuit excites a second fibre in the Einthoven galvanometer. Four photoelectric cell stations were used in a range of about 150 ft. The first was placed about 23 ft. from the muzzle, the others at equal intervals down the range. The accuracy was such that in measuring velocities of about 4000 ft. per sec. the probable error under the best conditions was of the order of five or six feet per second.