212 WRITINGS OF JOSEPH HENRY. [1843 



ON A METHOD OF DETERMINING THE VELOCITY OF PROJECTILES. 



(Proceedings of the American Philosophical Society, vol. iii, pp. 165-167.)* 



May 30, 1843. 



Professor Henry read a communication " On a New Method 

 of determining the Velocity of Projectiles." 



The new method proposed by the author, consists in 

 applying the instantaneous transmission of an electrical 

 action, to determine the time of the passage of the ball 

 between two screens, placed at a short distance from each 

 other, in the path of the projectile. For this purpose the 

 observer is provided with a revolving cylinder, moved by 

 clock-work at the rate of at least ten turns in a second; and 

 of which the convex surface is divided into a hundred equal 

 parts, each part therefore indicating in the revolution the 

 thousandths part of a second. Close to the surface of this 

 cylinder which revolves horizontally, are placed two galva- 

 nometers, one at each extremity of a diameter, the needles 

 of these being furnished at one end with a pen for making 

 a dot with printers' ink on the revolving surface. 



To give motion to the needles at the proper moment, each 

 galvanometer is made to form a part of the circuit of a gal- 

 vanic current, which is completed by a long copper wire 

 passing to one of the screens, and crossing it several times, 

 so as to form a grating, through which the ball cannot pass 

 without breaking the wire, and thus stopping the current. 

 During the continuance of the galvanic action, the marking 

 end of the needle is turned from the revolving cylinder a 

 few degrees, and pressed immovably against a "steady pin" 

 by the well known deflecting power of the electrical current ; 

 but the moment the current is stopped b}^ the breaking of 

 the long conductor, in the passage of the ball through the 

 screen, the marking end of the needle is projected against 

 the cylinder by the action of a fine spiral spring, similar to 



* [Re-printed in Walker's Electrical Magazine, 1845, vol. i, pp. 350-352.] 



