46 Dr. H. Pender on the Magnetic 



truth, and even under the best conditions the deflexion is 

 necessarily small. The following data, which are the mean 

 values from one set of readings, will suffice to illustrate the 

 capabilities of the method. 



d=l'60, ^ = -30, d 2 = '15. 



V = 24-9 c.G.s. electrostatic units (=7470 volts). 



a ='70 electrostatic units. 



A = 2'5l x 10 7 mms. per electromagnetic unit. 



N = 57-8. 



f r( Pl -p 2 )dr=22S. 



2D calculated 4*85. 



2D observed 4*5. 

 2D was the actual deflexion measured, not D. 



Considering the importance of the question as to the mag- 

 netic action of a moving static charge, the following experi- 

 ment, in which was observed the direct magnetic action of a 

 moving charged disk on a magnetic needle suspended near it, 

 may be of interest, though similar results have been obtained 

 by other experimenters. The uninjured one of the two 

 micanite disks used in the first experiments was provided 

 with a row of sixteen brass studs set at equal intervals apart 

 in a circle of 5 centimetres' radius around the centre of the disk. 

 The gilded surface of the disk was then divided into sixteen 

 sectors on each side, each pair of sectors carrying a stud. 

 The sectors were separated from one another by a strip of 

 micanite surface 1 cm. in width. The tinfoil on both the con- 

 densing-plates, with the exception of a sector on each twice 

 the width of a sector on the disk, was removed. The tinfoil 

 sectors were earthed, and the sectors on the disk could be 

 connected one at a time, through a brush B (fig. 3) set so as 



Fi°\ 3. 



rvi 



r u 



^Actual Size 



to make contact with the studs S, with one pole of the Voss 

 machine, the other pole of which was earthed. The frame 

 carrying the disk was so arranged that the disk could be set 



