466 Mr. J. M. Kuehne on the Electrostatic 



If T is the period of one complete oscillation, the angular 

 throw, assuming harmonic motion, is 



= To)_TNQ 

 TNCE' 



47r 2 ixyxio 12 ' 



where C is the capacity in electrostatic units and E' the 

 charging E.M.F. in volts. 



With a scale distance D the deflexion, as read by telescope 

 and scale, is 



DTNCE' 



d = 



2tt 2 1x9x10 12 



The first important difficulty encountered in the experiment 

 was due to inability to find any material for making the 

 movable cylinder and axis which was sufficiently non-magnetic 

 not to be violently affected by the enormous magnetic fields 

 used. Glass, wood, celluloid, aluminium, brass, and copper 

 rods were tried, and numerous specimens of mica, and later 

 paper, were used in making the cylinders. A thin rod of 

 copper wire, carefully freed from surface contamination by 

 leaving it in concentrated HC1 for a day or two, and cylinders 

 of pure unsensitized photographic paper, or good clear writing- 

 paper, proved more nearly satisfactory than anything tlat 

 was tried. With such a cylinder the purely magnetic de- 

 flexions could generally be brought within bounds, by care- 

 fully adjusting the position of the cylinder with reference to 

 the magnet, by means of the adjustable torsion-heads. 



A new difficulty arose when the cylinder was charged ; 

 for the two cylinders are in a position of unstable equilibrium 

 with respect to the electric attraction between them, and only 

 by having the suspension fibres stretched quite tight, and 

 then carefully adjusting the position of the outer cylinder, 

 was it sometimes possible to keep the suspended system 

 approximately balanced, and moderately free from magnetic 

 influence, while it was electrically charged. 



The experimental difficulties naturally increased with in- 

 creasing strength of magnetic fieid, magnitude of charge, 

 and sensitiveness of suspension. The largest total magnetic 

 flux attained was N = 300,000 lines, the highest charging 

 potential was E = 3000 volts, while the capacity, as calcu- 

 lated from the dimensions of the cylinders, was C = 10*2 

 electrostatic units. The other quantities entering into the 

 formula were: — T = 103 sec, I =18*2 g.cm. 2 ; and D = 318 cm. 



