22-26] Theories of Electrical Phenomena 19 



action of the matter on the electricity there must be an equal and opposite 

 reaction of electricity on matter. This, then, will act normally outwards at 

 the surface of the conductor. Experimentally it is best put in evidence 

 by the electrification of soap-bubbles. A soap-bubble when electrified is 

 observed to expand, the normal reaction between electricity and matter at 

 its surface driving the surface outwards until equilibrium is reestablished. 



25. Also when two conductors of different material are placed in con- 

 tact, electric phenomena are found to occur which have been explained by 

 Helmholtz as the result of the operation of reactions between electricity 

 and matter at the surfaces of the conductors. Thus, although electricity 

 can pass quite freely over the different parts of the same conductor, it is not 

 strictly true to say that electricity can pass freely from one conductor to 

 another of different material with which it is in contact. Compared, however, 

 with the forces with which we shall in general be dealing in electrostatics, it 

 will be legitimate to disregard entirely any forces of the kind just described. 

 We shall therefore neglect the difference between the materials of different 

 conductors, so that any number of conductors placed in contact may be 

 regarded as a single conductor. 



THEORIES TO EXPLAIN ELECTRICAL PHENOMENA. 



26. One-fluid Theory. Franklin, as far back as 1751, tried to include 

 all the electrical phenomena with which he was acquainted in one simple 

 explanation. He suggested that all these phenomena could be explained by 

 supposing the existence of an indestructible " electric fluid," which could be 

 associated with matter in different degrees. Corresponding to the normal 

 state of matter, in which no electrical properties are exhibited, there is 

 a definite normal amount of " electric fluid." When a body was charged 

 with positive electricity, Franklin explained that there was an excess of 

 " electric fluid " above the normal amount, and similarly a charge of nega- 

 tive electricity represented a deficiency of electric fluid. The generation of 

 equal quantities of positive and negative electricity was now explained : for 

 instance, in rubbing two bodies together we simply transfer " electric fluid " 

 from one to the other. To explain the attractions and repulsions of electri- 

 fied bodies, Franklin supposed that the particles of ordinary matter repelled 

 one another, while attracting the "electric fluid." In the normal state of 

 matter the quantities of "electric fluid" and ordinary matter were just 

 balanced, so that there was neither attraction nor repulsion between bodies 

 in the normal state. According to a later modification of the theory the 

 attractions just out-balanced the repulsions in the normal state, the residual 

 force accounting for gravitation. 



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