RESISTANCE BY A METHOD BASED ON THAT OF LORENZ. 
93 
Section 23.— Determination of Resistance in Absolute Measure. 
(l) Preliminary Tests. 
(A) Elimination of Error due to the Thermo-electric Effects at the Brush Con¬ 
tacts .—-The thermo-electric effects are eliminated by observing only the change of 
deflection of the galvanometer with reversal of the current in the coils. If the 
magnitude of the changes of the thermo-electric effects is considerable, and great 
accuracy is desired, a large number of reversals are necessary. It becomes important, 
therefore, to determine the general magnitude of the error introduced by making a 
comparatively small number of reversals. 
A, variations in thermo-electric effects; B, variations in thermo-electric effects plus variations in speed. 
Our normal procedure in making resistance measurements was to reverse the current 
in the coils at intervals of 15 seconds, and in practically all cases 48 reversals were 
made. To determine to what extent this procedure eliminated the thermo-electric 
effects, we operated the reversing switch with no current flowing in the coils and 
noted the galvanometer deflection at the end of every 15 seconds. 
The differences obtained in one such experiment are plotted against time in graph 
(A) fig. 24. The mean difference from the mean is ± 0‘03 mm., which would intro¬ 
duce an error in a measurement of resistance of something less than 1 part in 
