WIRE BRIDGE. 



365 



Contact at C is effected by means of a kind of rounded knife- 

 edge, usually of platinum, which is applied perpendicularly on the 

 wire. This mode of contact would be very defective in the case 

 of a rheostate, but does very well here ; for its resistance does not 

 come into play, and it is only necessary to determine exactly the 

 position of the point touched. 



Great care must be taken to avoid any alteration or deformation 

 of the wire. With this view, great attention must be paid to the 

 mechanical arrangements for making contact. For the same reason 

 the battery must not be placed between the points D and C, in order 

 not to alter the surface of the wire by sparks on breaking contact. 



Fig. 192 represents a bridge of a very careful construction, made 

 by M. Carpenter, for reproducing the ohm. The scale is of brass, 

 and is divided in millimetres. It serves to put the movable contact 

 in connection with the galvanometer, which dispenses with the use 



of a flexible wire. The contact is made by a steel knife-edge, which 

 is raised in its ordinary position. By pressing an ebonite key M, 

 the knife itself is not acted on, but it is left to the action of a small 

 spring which produces on the wire a slight pressure, always equal 

 and independent of the operator. This knife-edge is supported by 

 a roller which moves along the scale, and can be fixed by a clamping 

 screw and adjusted by a two-way screw. A vernier on the roller 

 gives fractions of division of the scale. The wires P and P' go 

 to the battery, and the wire G and G' to the galvanometer. 



The resistances a and , which are represented on the figure 

 by a copy A of the ohm, and a coil B contained in a copper 

 cylinder, are introduced into the circuit by means of copper mer- 

 cury cups fixed on lateral bands. They can thus be very easily 

 replaced for each other. The resistances a' and V are also en- 

 closed in the same cylinder C. The contact is effected in the 



