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XCIII. Unipolar Induction. By E. H. Kennard "*. 



ONE of the chief characteristics of Lorentz's Electron 

 Theory is the assumption that electromagnetic pheno- 

 mena depend in part on motion through an sether which is 

 everywhere at rest, but this principle has not yet been tested 

 by any method which employs only quasi-stationary pheno- 

 mena. It is, however, involved in a simple manner in 

 unipolar induction, and this would seem to render a definite 

 solution of that much-debated problem very desirable. 



In the classical form of unipolar induction discovered by 

 Faraday, a bar-magnet (M) (fig. 1) is rotated about its axis of 

 magnetic symmetry, while a wire leading to a galvanometer 



Fig. 1. 



(G) makes sliding contact with it at one end and in the 

 middle. In the circuit so formed an E.M.F. is developed 

 whose amount is always given by Faraday's law of induction. 

 Two views as to the seat of this E.M.F. have been held : (1) 

 that it is produced in the wire, which is at rest, as the magnetic 

 force lines rotating with the magnet cut through it ; {2j that 

 it is produced in the magnet itself as the latter moves through 

 its own magnetic field. It is clearly impossible to decide 

 between these two views by any observations on closed 

 circuits ; hence, in the present investigation an electrometer 

 and an open circuit were employed. 



Fig. 2. 



The apparatus is shown diagrammatically in fig. 2. A steel 

 bar (B), 2 feet long and 2 inches in diameter, was mounted 



# Communicated by Prof. J. S. To-wnsend, F.R.S. 



