648 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



and then interferes with the motion and stops the clock. The result 

 of this arrangement is that if the current ceases to act when the 

 projectile starts, and is re-established when it strikes the target, 

 the clock will go only during the transit the precise duration of 

 which it will consequently indicate. 



This condition is realised in the following manner. The battery 

 p communicates on one side with the chronoscope, and on the other 

 side with the target M, and by a connecting wire with the cannon 

 c the wire / passes in front of the mouth H of the piece. 



A little before commencing the experiment the derived circuit is 

 closed, and the current passes ; and now the clock is stopped. The 

 command to fire is then given, the wire is cut by the ball, the circuit 

 is broken, the clock, let free, goes on until the moment when, by striking 



Firt. 424. Wheatstone's clmmoseope. 



the target, the projectile brings into contact the two wires that are 

 attached to it, and closes the circuit again. The clock is now stopped 

 again, and the position at this moment of the needles on the two 

 dials indicates, in seconds and fractions of a second, the exact duration 

 of the flight. 



Wheatstone himself perceived the disadvantages of this first 

 arrangement. The magnetism remaining in the armature caused by 

 the contact was maintained a little after the rupture of the current ; 

 on the other hand the motion of the needles was not immediately 

 arrested upon the impact of the shot upon the target, and however 

 small these differences might be, they were sufficient to render uncer- 

 tain the indications of the chronoscope, especially for such small 

 fractions of a second. The inventor was able to correct in some 



