492 On rotating a Dielectric in a Magnetic Field. [May 18, 



The mean result obtained for the quantity of electricity set free on 

 the outside coating of the cylinder, on reversing the magnetic field, 

 only differs from the amount calculated theoretically by 1 per cent. 

 The specific inductive capacity of the ebonite, as determined by 

 measuring the capacity of the cylinder, was 3-54, while the value 

 calculated from the results obtained, using the formula E = nirK 

 (r 2 2 -ri 2 ) (1 -K -1 ), was 3*64. As the result of these experiments, it 

 may be concluded that — 



1. A radial electric displacement is produced in a dielectric, such as 

 ebonite, when it is rotated in a magnetic field parallel to the axis of 

 revolution. 



2. The direction of the displacement is the same as is produced in a 

 conductor. 



3. The displacement is proportional to the magnetic field, and to the 

 rate of revolution. 



4. The amount of the displacement agrees with that calculated on 

 the assumption that the induced E.M.F. in the dielectric is equal 

 to that in a conductor multiplied by 1 - K" 1 . 



The results obtained are thus in complete agreement with the 

 theories of Lorentz and Larmor, and may be regarded as a confirma- 

 tion of these theories. 



The experiments were done in the Cavendish Laboratory, and the 

 cost of the special apparatus used has been defrayed by a grant from 

 the Royal Society. 



