INTRODUCTORY. 



resultant of all the actions which each of the elementary masses, 

 considered separately, would exert upon it ; and this whether the 

 masses belong to separate bodies, or whether they form part of the 

 charge of one and the same body. 



For the sake of abbreviation, we shall apply the term, electrical 

 force at a point, to the resultant of all the actions which would be 

 exerted on unit mass of electricity placed at this point. 



8. DISTRIBUTION OF ELECTRICITY. Coulomb proved by direct 

 experiment that when an electrified conducting sphere is put in 

 contact with an identical sphere in the neutral state, each of them 

 possesses a mass of electricity equal to half the original mass that 

 is to say, that each of them acting separately at the same distance 

 upon an external electrified body, exerts half the action of that of 

 the sphere in its original state. If the same sphere, instead of being 

 neutral, is itself electrified before contact, the final charges are also 

 equal ; each of them is half the algebraical sum of the original 

 masses, so that it is zero, and the bodies are in the neutral state, 

 if the initial charges were equal, and of opposite signs. 



This would also be the case with two identical conductors of any 

 given shape, which were made to touch, provided that they were 

 symmetrical at the point of contact. 



If the condition of symmetry be not fulfilled, the charges are no 

 longer equal ; but their algebraic sum is always equal to that of the 

 original mass. This is a general fact, and applies to any number of 

 bodies, however they may be placed in relation to each other ; and 

 provided that none of the conductors are put, even for a moment, in 

 contact with the earth, the algebraical sum of the electrical masses of 

 the system remains the same. 



9. ELECTRICITY OF CONTACT. Volta discovered this most impor- 

 tant fact, that the contact of two different metals, originally in the 

 neutral state, or more generally, of any two bodies at the same 

 temperature, is sufficient to place them in two different electrical 

 states, and to charge them respectively with equal quantities ot 

 electricity of opposite signs. 



Friction is only a particular case of contact. The cause which 

 produces the electricity seems then to be the same in both cases. 



It follows from Volta's discovery, that two conducting spheres of 

 the same radius would only have equal charges after contact, pro- 

 vided they were of the same material, and at the same temperature. 

 But this is no exception to the fundamental proposition, that the 

 algebraical sum of the charges is the same before and after contact. 



10. ELECTRIFICATION BY INFLUENCE. INDUCTION. When a 



