1904.] Rotating a Dielectric in a Magnetic Field. 491 



electrometer ; the quantity of electricity required to produce a given 

 deflection of the electrometer needle being determined by means of a 

 small parallel plate guard ring condenser. 



It is shown that if E is the induced E.M.F. in the ebonite, and V 

 the potential difference indicated by the electrometer, then 



2E = V c + c ' 

 c 



where c is the capacity of the ebonite cylinder and c' the capacity of 

 the electrometer. If the electrometer gives a deflection D due to V,. 

 and a deflection d due to a known quantity of electricity q, then 

 2Ec = Y(c + c') = ~Dq/d. Consequently E can be determined in terms, 

 of c, D, d and q. 



If the outside radius of the ebonite cylinder is r 2 , and the inside radius 

 r v then according to the electromagnetic theory E = mrH (r 2 2 - ?*i 2 ) 

 (1 - K _1 ) where H is the strength of the magnetic field and n the number 

 of revolutions per second. 



The cylinder used was 10 cm. long, and 2r 2 = 4*15 cm., 2ri = 2'01 cm. 

 It was mounted in a solenoid having 95 turns per cm., by which a 

 magnetic field of strength 1500 could be produced. The cylinder was 

 driven by a J horse-power motor, and could be run at 200 revolutions 

 per second. The following table contains the results obtained with 

 various rates of revolution and magnetic fields. 



Kevolutions 

 per second. 



Magnetic field 

 reversed. 



Deflection of 

 Electrometer 

 due to one 

 E.S. unit. 

 (Millimetres.) 



Deflection. 

 (Millimetres 

 observed.) 



! 



Deflection. 

 (Calculated). 



i 



192 



1480 



6015 



26 '2 



26 -0 



192 



750 



6015 



13-6 



13 -2 



192 



380 



6015 



6-8 



6-7 



192 



320 



6015 



5 -3 



5-6 



183 



1420 



4780 



18 -0 



18-9 



182 



710 



4780 



9-2 



9-4 ' 



182 



1160 



6040 



19-9 



19 4 



100 



1400 



6320 



14 -0 



13 -5 



100 



1100 



6320 



10 *5 



10-6 



100 



670 



6320 



7-0 



6-4 



100 



540 



6320 



5-0 



5 2 



93 



1100 



6100 



9*5 



9 5 



92 



1460 



4770 



10-0 



9-7 



92 



1260 



4770 



8-0 



8-4 



92 



650 



4770 



4-5 



4-3 



49 -2 



1200 



6320 



6'0 



5-7 



49-2 



600 



6320 



3 



2-8 



