THE ROYAL ARTILLERY INSTITUTION. 
173 
copper c forms a connexion between the staples b and d\ the copper e joins 
d and f y and so on. A galvanic stream will therefore take the following 
course, whether the springs be weighted or unweighted : copper a, brass b , 
copper c, brass d } copper e , brass f 9 copper g , &c. The current will only be 
interrupted when one or more threads have been cut and the corresponding 
spring is flying from the bottom to the top of its hole. About ^Vth of a 
second is required for the complete registration of such an interruption, the 
spring traversing about half an inch. The shelf B is placed for the weights 
to rest against, partly to prevent them from being carried forward by the 
shot, but chiefly to prevent the untwisting of the threads which support the 
weights. The weights used were about 2 lbs. each, and the strength of the 
sewing cotton for supporting them was equal to a stress of about 3 lbs., 
which was sufficient to withstand a tolerably strong wind. As the weights 
were equal the threads were kept equally stretched. 
The arrangement of the screens for an experiment is shown in Tig. 5. 
The wires for conveying the galvanic current are, like the common telegraph 
ware, carried on posts, abc is a continuous piece of wire; but there are 
interruptions between e and h , between i and l , between m and p, &c., in 
order to make the galvanic current circulate through the screens. The 
course of the galvanic current is + a, b, c, d } e, f } g, h, i, j, k, l , m, n, o, 
Pi B r ) The ends a , t , are connected with the instrument and battery. 
The shot, being fired through the screens, in passing cuts one or more 
threads at each screen, so that corresponding to the instant at which the shot 
passes each screen there is an interruption of the galvanic current, and a 
simultaneous record on the paper. 
^VJ/hen the cylinder is filled with spirals, that is after five or six shots, it 
is transferred to the instrument, Eig. 6, where a is a circle divided into 300 
equal parts, and the division is carried to 3000 by the help of a vernier* 
A small T-square, having a fine edge at b> moves along a brass straight-edge 
Jj, adjusted so as to be parallel to the axis of the cylinder* The mark b is 
[vol, v.] 23 
