﻿938 



Mr. E. H. Kennard 



on 



inside a solenoid (S) by means of which it could be magnetized 

 at will, and the whole supported about 4 feet off the floor. 

 The bar, which will hereafter be referred to as the magnet, 

 was set in rotation by means of a motor. Concentric with 

 the solenoid was placed an insulated metal cylinder (C) 

 shorter in length but of larger diameter. The solenoid and 

 all connected parts, including the motor and belt, were 

 surrounded with a continuous metal case (K) , which was put 

 to earth and served to protect the outer cylinder from electro- 

 static disturbances originating inside the case. The outer 

 cylinder was connected with a quadrant electrometer (E) 

 placed nearly in line with the axis of the magnet. 



The outer cylinder was about 49 cm. long and 20 cm. in 

 diameter. The case surrounding the solenoid, which will be 

 referred to as the "inner cylinder/' was 7*3 cm. in diameter. 

 The electrometer scale gave very nearly 850 divisions per 

 volt. 



In this apparatus either the inner cylinder, which was 

 earthed (Z) and therefore communicated with the end of the 

 magnet through the axle, or the outer cylinder and the wire 

 leading from it to the electrometer and so to earth (when the 

 electrometer was earthed) and back to the end of the magnet, 

 might be regarded as corresponding to the wire in Faraday's 

 experiment, only the sliding contact at the centre and a 

 short connecting wire being omitted. Hence, if in Faraday's 

 experiment the E.M.F. was produced in the wire, it will here 

 be produced in the cylinders and connecting wires; these 

 will then come to some electrostatic potential different from 

 that of the earth and the outer cylinder will be charged. If, 

 on the other hand, the E.M.F. is produced in the magnet, it 

 could influence the outer cylinder only through electrostatic 

 induction, and such effects are screened off by the earthed 

 metal case. Thus, by observing the effect on the electrometer, 

 it is possible to determine the seat of the E.M.F. 



The first view corresponds to what may be called the 

 " Moving Force Line Theory." According to this theory 

 the electromotive intensity E' at any point in the circuit is 

 given by 



E' = E+i[V'H], 



where E is the electric force and [V'H] is the vector product 

 of V, the velocity of the circuit " relative to the magnetic 

 force-lines," i. <?., to axes fixed in the magnet, and H is the 

 magnetic force. This theory secures complete relativity, that 

 is, the phenomena depend only on the relative translation or 



