DowLiNG AND DoNNELLY — Measurement of Short Time Intervals. 167 



practically the same degree of tension before soldering. These were broken 

 in turn by a small (•22) rifle bullet travelling in the direction of the arrow, 

 and the velocity of this bullet was measured immediately afterwards by an 

 independent method. Check observations showed that the wires produced no 

 appreciable retardation of the bullet. In the experiments the wires were 

 never mounted closer than 2 cms. apart, so that the shghtly different lateral 

 displacements which occur before breaking should not seriously affect the 

 result. 



The bullet velocity was determined as follows : — Two circular discs of 

 cardboard, about two feet in diameter, were mounted about two feet apart, 

 one at either end of a stout shaft, turning between centres, and driven by a 

 powerful electric motor at a speed of about 35 revolutions per second. 

 To these discs somewhat larger thin paper discs were pasted, and the whole 

 apparatus mounted so that the bullet, travelling parallel to the axle, would 

 pierce the two paper discs in succession near their edges. A resistance in the 

 motor circuit allowed the speed of the discs to be adjusted to a constant 

 steady rate indicated by a stroboscope arrangement. The shot was then 

 fired, and from a measurement of the angular displacement of the bullet 

 holes, relative to each other, the bullet velocity could be determined. Certain 

 obvious check experiments were carried out to determine whether, for 

 instance, the bullet was appreciably retarded or deflected sideways during its 

 passage through the first disc. The angle to be measured was generally about 

 30°, and an error of more than one or two per cent, was unlikely. 



The apparatus was connected up as shown by the /;;// lines in the diagram. 

 (The dotted lines P, Q, r refer to a modification referred to later.) Observations 

 were conducted as follows : — Having measured the separation of the wires in 

 position, and both keys Ki, Kz being open, the motor speed was regulated until 

 the standard speed of rotation of the discs was reached. The gun was imme- 

 diately fired, breaking the wires A and B. The shunt being set to unity, A', 

 was closed and the deflection observed for q. To obtain the reading for q^,, 

 Ki and Ki were then operated in succession, the shunt having been set back 

 to ' 10.' The observations of the angular displacement of the bullet holes in 

 the discs were finally taken by means of a protractor attached to the axle, 

 the punctui'es being sighted in turn through the rifle barrel. The following 

 table contains the results of twelve consecutive readings obtained in this 

 manner. 



In all cases rate of rotation is 34'5 revolutions per second, and distance 

 apart of paper discs 54"8 cms. 



