III. On the Elect in- Infect of Rotating a Dielectric in a Magnetic fr'ield. 



fry HAROLD A. WILSON, M.A., D.Sc., Fellow of Trinity College, Cambridge. 



Communicated by Professor J. J. THOMSON, F.li.S. 



Received May 18, Rewl June 2, 1904. 



IT was shown by FARADAY in 1831 that an electromotive force is induced in a 

 conductor moving in a magnetic tield so as to cut the lines of magnetic force. The 

 object of the experiments described in the present paper was to see if a similar effect 

 exists in a dielectric when it moves in a magnetic field, and to measure the amount 

 of the effect if it were found to exist. 



According to MAXWKLL'S electnxlynamic theory, as developed by H. A. LORENTZ 

 and by LARMOR, such an electromotive force should be produced in a dielectric, and 

 should be equal to the electromotive force in a conductor multiplied by the factor 

 1 K" 1 , where K is the specific inductive capacity of the dielectric. 



It appears, from the experiments described below, that the effect in question does 

 exist, and is very nearly of the amount predicted by the theory. The experiments, 

 therefore, may be regarded as a confirmation of the theory in question. 



The paper is divided into the following parts : 



(1) Theory of the experiment; (2) description of apjwratus ; (3) method of 

 experimenting and results obtained ; (4) comparison of the results with 

 theory ; (5) summary of results and conclusion. 



(1.) Theory of tfif Experiment. 



In FARADAY'S original experiment a metal disk was placed in a magnetic field 

 perpendicular to the plane of the disk, and the axle of the disk and its circumference 

 were connected by sliding contacts to a galvanometer. When the disk was rotated, a 

 current passed through the galvanometer, showing that an electromotive force was 

 induced in the disk by its motion in the magnetic field. 



The method I have used is very analogous to that employed by FARADAY. A 

 hollow cylinder of dielectric is rotated about its axis in a magnetic field parallel to 

 the axis of the cylinder, and the inside and outside surfaces of the cylinder are 



VOL. rciv. A 374. R 27.10.04 



