40 Dr. H. Pender on the Magnetic 



this opportunity to call attention to two slips made in the 

 printer's copy of my former paper, namely, the symbol ir for 

 V in the formula on pp. .199 and 200, and 4 for 8 in the 

 formula at the bottom of p. 197.) In deducing this formula 

 the assumption was made that the charge on the disks was 

 reversed from a positive to an equal negative value and vice 

 versa by the reverser. In all the experiments here described 

 the disks were alternately charged and discharged, sometimes 

 negatively and sometimes positively. Hence to apply to these 

 experiments the above formula must be written 



2V.NA r , vB . / 7r/3\n 



v =XB^mi^^ g r cos 2B/J- 



A is the deflexion produced by reversing unit current in 

 the test-coil at the same rate as the disks are charged and 

 discharged. Hence if d is the deflexion produced by the 

 current i under the same conditions, 



A = <* 



If the potential is measured in volts and the current in 

 amperes the above formula then becomes 





2VNd r , B . / 7r/3\l 



30CB^8pr + V 7r lo H 2COS ^)J- 



fjb and v were determined in the manner described in the 

 former paper. The distance between the condensing plates 

 was kept constant. While the disks were running at a high 

 speed one day in the early spring the east disk flew off the 

 axle and was so badly damaged that it could not be used 

 again. Instead of waiting to have it repaired, which would 

 have taken considerable time, I went ahead with the remain- 

 ing disk, making a virtue of necessity by thus varying the 

 conditions of the experiment. 



The constants in the above formula are as follows : — 



Mean value for the two disks. Value for the west disk. 



B= 2-432 B= 2-469 



£= -356 /3= -356 



^=115-0 /*= 115-0 



v= 28*8 v= 28-8 



It so happened that the two disks were arranged almost 

 perfectly symmetrically with respect to the coil, so that fi 

 and v for the two sides were identical within the limits of 



