ver 



•y small Intervals of Time. 



339 



Fig-. i. 



-UP 



jk: 



Vi 



] i 



3!~=r 



e 1 JL- 



which have to be broken are a 

 and b, — first a, then b. In the 

 figure, a and b are supposed to 

 represent, byway of illustration, 

 two thin wires stretched at dif- 

 ferent distances fromthe muzzle, 

 B, of a gun in such a way that 

 the shot passes through them 

 in the direction of the arrows. 

 The battery E has so little re- 

 sistance* in comparison with 

 that of the discharging wire r) 

 that the potential of the accu- 

 mulator, A, remains practically 

 unaltered when r is made infi- 

 nite. 



A key of peculiar construc- 

 tion is convenient for observing 

 he initial charge. It consists 

 of a lever (K) turning on pivots 

 at 4. When in its position of rest (as shown in the figure) 

 the rigid contact, 1, at the top in front, and the spring contact, 

 2, underneath at the back, are both in connexion with the lever. 

 When the knob is depressed the rigid contact 1 is first inter- 

 rupted; an instant afterwards the spring contact, 2, is inter- 

 rupted; and lastly the front contact, 3, is made. This arrange- 

 ment ensures the removal of the discharging resistance r, before 

 the interruption of the battery, by the key. 



Depressing for a moment the knob of the key, the resistance 

 and battery are therefore successively interrupted, and the in- 

 stantaneous discharge from the accumulator is read off by the 

 excursion of the galvanometer-needle at Gr. This gives the 

 value C. 



Letting the key resume its position of rest and recharge the 

 accumulator, the wires a and b are in turn interrupted by firing 



shot through them. The interruption of a puts the bat- 

 tery out of action ; and the charge which is in the accumulator 

 at the moment commences to flow out through the discharging 

 wire r. This flow is stopped as soon as the shot reaches and 

 breaks b. The observer, as soon as he hears the report, de- 

 presses the key and reads the excursion (c) of the needle due 

 to the remainder of the charge. 



The accuracy of this method depends upon the conditions 

 (1) that the difference of potential between the two sides of 

 the accumulator is not altered practically by its poles being 

 * A single cell of Daniell's battery of large surface is sufficient. 

 2 A2 



