THE MEASUREMENT OF RESISTANCE 157 



D x and D s are the readings, and R x and R s the resistances 

 unplugged in R when the terminals are on X and on S respec- 

 tively. Ry is the voltmeter resistance. If the current fluctuates, 

 two voltmeters should be used, simultaneous readings being 

 taken by two observers. This procedure, millivoltmeters being 

 employed, is frequently used for testing rail bonds in situ; the 

 resistance of a given length of rail including a bond being com- 

 pared with that of the same length without a bond. The volt- 

 ages measured are those due to the return current through the 

 rail. 



Potentiometer Method. Instead of determining the potential 

 difference between the terminals of S and of X by a voltmeter, 

 the potentiometer (see page 271) may be used. Obviously the 



V.M. 



FIG. 83. Direct deflection method for measuring resistance. 



current in S, in X and that in the potentiometer must remain 

 constant during the test. As the processes of balancing and 

 checking the constancy of the potentiometer current require some 

 time, this condition is very difficult of practical realization; while 

 the method may be made to give accurate results, the measure- 

 ment becomes a time-consuming operation. In very accurate 

 measurements, the possibility of a heating error is considerable, 

 for the current must be kept on continuously during the process 

 of balancing. 



In order that resistances may be determined with accuracy and 

 despatch, it is necessary to have methods which are independent 

 of fluctuations of the testing current. Such methods will now be 

 discussed. 



Differential-galvanometer Method. A differential galva- 

 nometer has two distinct windings which are thoroughly insulated 

 from each other, of equal magnetic strength, of equal resistance, 

 and as nearly coincident as possible. To attain these conditions 



