SCIENCE AND PRACTICE. 309 



bridge may be used for other than copying purposes, by in- 

 serting proportion resistances between a a and c c , the com- 

 parison resistance or set of adjustable coils between s and s' 

 and the resistance to be measured between r and r. As the 

 resistance of the wire, w x, might be embarrassing in the 

 general employment of the bridge, it may be short-circuited 

 by a wire between the mercury cups, e and e. 



52. Balance formed by a Bisected Wire. Balances, the two 

 proportion resistances of which are made variable by moving 

 the point of bisection of a wire, are the most delicate and 

 best adapted for measuring very small resistances ; they are, 

 of course, of more use in the physical laboratory than in the 

 testing-room. 



Fig. 148 represents a perspective view of the wire bridge 

 constructed by Dr. Siemens for use in his laboratory at 

 Berlin, and with which the mercury unit of resistance was 

 determined, and most of the elegant experiments made by 

 that able physicist carried out. 



Upon three thick slabs of vulcanite, a a, a a', and a" a t 

 which rest upon a table, is supported the brass guide, A A, 

 on the top of which is a rack, D D, whose teeth engage 

 with those of a horizontal pinion underneath the carriage 

 B B, with which it travels when the milled head c is 

 turned to the right or left. The same slabs of vulcanite 

 support a brass scale, m m, a metre long, graduated in 

 millimetres ; in front of the scale is stretched, between 

 insulated metal clamps, a fine platinum wire, w w, of 

 exactly a metre in length between the clamps, passing 

 between two platinum contact rollers, G, carried by B B. The 

 clamps, between which the ends of the wire are held, are in 

 metallic connection with the bolts and couplings, E E, and 

 from these through thick copper bars to the opposite corners, 

 1 and 2, of a commutator, s. The other corners, 3 and 4, of s 

 are connected by similar bars with the clamps K K in front. 

 In the clamps K K are sliding connecting rods, L L, between 

 which and the front contact H of a key, J, the standard re- 

 sistance w, and the resistance x, which is to be measured, 

 are connected* 



