96 ELECTRICAL MEASUREMENTS 



start, otherwise confusion may arise and the results be of no 

 value. 



The procedure is as follows: Observe, if necessary, the tem- 

 perature of the conductor. With K open, simultaneous readings 

 of the millivoltmeters are taken. Denote them by Fi and F 2 , 

 then 



K is then closed and simultaneous readings of the ammeter and of 

 the two millivoltmeters are taken. Call the readings 7, F'i, F' 2 , 

 and denote the stray current in the pipe at the instant of reading 

 by /,. Then the current across x, at that instant, will be 



/*=-/ + /, 

 also 



I x x = F'x 



Lr -- F' 2 

 From these 



= - Ix + I a x = - Ix 



F/ F' 2 Fi 

 and 





F 2 F/ F^ 2 

 VJ I' 



The test should be repeated with the battery reversed; I is 

 then . Throughout, care must be taken as to the algebraic 

 signs of all the deflections. 



The final result is independent of the stray current in the pipe, 

 /,; its elimination is possible, even though it be varying rapidly, 

 because all three instruments are read simultaneously. The 

 periods and the damping of the three instruments must be such 

 that they keep pace with one another when the current changes. 



It will be noted that if the current 7 is so adjusted that the 

 reading of F'i becomes zero, 



