130 



DR. HAKOIJ) A. WILSON ON T1IK KLKCTKIC EFFECT OK 



putting oil on the brushes, which was thrown off hy the centrifugal force and caused 

 a very rapid drift in the same direction as that obtained when the brushes \\riv not 

 pressed down enough. By carefully adjusting the bearings and brushes the index 

 was got to remain very fairly steady on a number of occasions and the measurements 

 recorded below were obtained. The pressure of the brushes was adjusted by hanging 

 small weights on the levers. 



If the index was drifting slowly at a uniform rate, then measurements could still 

 be made by taking the deflection first in one direction and then in the other, and 

 taking the mean of the two as being the true deflection. Measurements got in this 

 way agreed with those obtained when the index was steady, but the measurements 

 given in the table below were obtained with the index very nearly steady, so that the 

 deflections were nearly the same in either direction. 



For example, on one occasion a current of 7 '5 amperes on reversing gave a deflection 

 of 13 '8 millims. one way and 13'0 millims. the other way. The index then began to 

 drift and the deflections became 25'0 millims. one way and 2 '5 millims. the other way. 

 The mean of the first pair is 13 '4 millims. and that of the second pair 13 '7 millims. 



On several occasions the index remained almost steady for some minutes and it was 

 verified very carefully that the deflection on reversing the current was a permanent 

 one and not merely a throw. The variation of the scale reading with the time on one 

 occasion is shown in fig. 5. The magnetic field was put on at A, reversed at B and C, 



eoo 



and taken off at D. It will be seen that between B and C a change in the zero 

 occurred which is indicated approximately by the dotted line. This diagram (fig. 5) 

 indicates the sort of results that could usually be obtained when the apparatus was 

 working fairly well. But, as already mentioned, on several occasions the index 

 remained quite steady for some minutes, and many of the results recorded below were 

 obtained on one or another of these occasions. 



