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XLIV. On a Model illustrating Mechanically the Passage of 

 Electricity through Metals, Electrolytes, and Dielectrics, ac- 

 cording to Maxwell's Theory. By Oliver J. Lodge, B.Sc* 



[Plate III.] 

 § 1. nnHE obstructions which an electric current meets with 

 -*- in its passage through a medium are of two kinds, 

 viz. opposition electromotive force and resistance proper. The 

 first, since it does not necessarily change sign nor vanish with 

 the current, has a tendency to produce a countercurrent. The 

 second opposes the current with a force which is always pro- 

 portional to the current and always acts against it; it can 

 therefore produce no reverse effects. 



Since the laws of the now of electricity are in most respects 

 the same as those of an incompressible fluid moving in a closed 

 circuit, we may represent a current of electricity in a conduct- 

 ing medium by a flexible inextensible endless cord circulating 

 continuously over pulleys. When the cord is at rest in the 

 conducting medium, it is no more (and no less) to be called 

 electricity than an ordinary copper wire is to be said to con- 

 tain electricity at rest ; but when it is in motion it is to be 

 called an electric current, and the strength of the current 

 through unit area of the conductor is to be measured by the 

 quantity of cord which passes any fixed point in a second ; in 

 other words, the strength of the current is proportional to the 

 velocity of the cord. The cord can be made to move by ap- 

 plying to any part of it, in the direction of its length, a force 

 which corresponds to electromotive force, and which can be 

 generated in various ways, as by a winch, a weight, or an 

 elastic string. 



To represent the resistance of conductors, the cord may be 

 made to rub against rough surfaces ; and to represent opposi- 

 tion electromotive force, the motion of the cord may be made 

 to stretch or to squeeze elastic bodies, thereby setting up a ten- 

 sion in them which shall not only oppose the motion, but shall 

 also have a tendency to force the cord in the reverse direction. 

 The difference of potential between any two points of a cir- 

 cuit is represented by the difference between the tensions of 

 the cord at the two points ; a greater tension is to represent 

 negative potential, while a less tension or a pressure is to re- 

 present positive potential. 



If the cord passes through a body, and if by any means we 

 can make that body include more of the cord than its normal 

 allowance, then that body is said to be positively charged with 



* Communicated by the Author, having been read before Section A 

 of the British Association in Glasgow, September 1876. 



Phil. Mag. S. 5. Vol. 2. No. 12. Nov. 1876. 2 A 



