THE MEASUREMENT OF RESISTANCE 



193 



that the resistances of the connecting leads cc', dd', ee' and //' 

 are so small, compared with the resistance with which they are 

 in series, that their effects are negligible. 



With the Thomson bridge as it is actually used, X, P and a are 

 always small, so that it is necessary to control the bridge current 



O 



FIG. 109. Variable standard resistance for use with Thomson bridge. 



by a rheostat; the current should be as large as is compatible with 

 accuracy. Its limiting value, and therefore the sensitivity of 

 the bridge, is fixed by the carrying capacity of the 'resistances 

 employed; consequently ample provision must be made for dis- 

 sipating the heat generated in the various arms. 



Rheo. 



FIG. 110. Diagram of laboratory form of Thomson bridge. 



The resistance to be measured must be provided with potential 

 terminals or their equivalent. In dealing with rods the contacts 

 at c and d may be made by soldering small wires across the rod, 

 the superfluous solder being carefully removed, or more con- 

 veniently, by point or knife-edge clamps. 



13 



