190 



ELECTRICAL MEASUREMENTS 



MEASUREMENT OF LOW RESISTANCE 



Wheatstone Bridge Method. When the value of X becomes 

 small, 0.1 ohm or less, it is difficult to determine it accurately, 

 owing to the uncertainty due to contact resistances introduced 

 at the binding posts where X is clamped to the bridge. For 

 example, such resistance for a No. 12 wire clamped by a binding 

 post may be about 0.00015 ohm. Again, all standard low 

 resistances and the shunts used in current measurements are 

 provided with potential terminals, and the desired resistance is 

 that between the points where these terminals are connected 

 to the main resistance. The following method of comparing 



such resistances with a stand- 

 ard is very useful, since it re- 

 quires no special apparatus. 



Let it be required to meas- 

 ure the resistance of a given 

 length of the bar X. Con- 

 nect it in series with the 

 standard resistance, S, as in- 

 dicated, and attach two po- 

 tential terminals, c and d, at 



bridge method ^ pr P6r P tS , (FIg " 107) ' 

 for comparing low resistances. L h e s e terminals may be 



clamps making contact with 



the bar by a pointed screw or knife edge. In order to compare 

 X and S, the intermediate resistance between b and c must be 

 eliminated. To do this connect the galvanometer at b and 

 balance; call the necessary value of R, R b) then 



r>i or 



ZL O 



R b X + a 



Now change the galvanometer wire to c and balance again; call 

 this value of R } R c , then 



R c X 



a can be eliminated from these two equations, giving as a result: 



