4 John Mills 



support. To this a lever arm was attached. Applying a slight 

 pull to this arm produced a torque on the support which varied 

 the length of path of one beam, and so caused the fringes to move 

 across the field. By this slight motion their presence could be 

 recognized after they were otherwise indiscernible. 



A series of these readings appears below. In the following 

 table it is to be noted that the mean of the first two columns 

 would give the current flowing in each coil. Twice this mean 

 would give the current necessary to produce the same effect if 

 only one coil was causing rotation. 



TABLE I— For a Rotation of 90° 



TABLE II— For a Rotation of 270° 



148 



