ELECTRICAL FORCE. 



The electrical mass of a body, other things being equal, is (pro- 

 portional to the force which it exerts upon an external body placed 

 at a distance) considerable in reference to its dimensions ; and the 

 mutual action of two electrical masses is proportional to the product 

 of their electrical masses. 



5. When two bodies for instance, a glass disc and one of metal, 

 the latter being insulated are kept in contact after being rubbed 

 together, the whole system behaves in reference to an external body, 

 electrified or not, just as if it were in the neutral state. The elec- 

 trical properties developed by friction have not, however, disappeared ; 

 for if the two discs are separated, it may at once be shown that each 

 of them is electrified. The actions of the two bodies in contact are 

 accordingly equal and of opposite signs. Hence follows this double 

 conclusion. 



By their mutual friction two bodies acquire quantities of electricity^ 

 which are equal and of different kinds. 



The law according to which the action varies with the distance is 

 the same for the two electricities. 



6. ELECTRICAL SIGNS. We are thus led to consider electrical 

 masses of different kinds, as quantities of the same nature and of 

 opposite signs. When a closed surface contains electrical masses of 

 different kinds, the action exerted upon an external mass, equal to 

 unity, and at a great distance, as compared with the surface in 

 question, is proportional to the difference of the electrical masses of 

 each kind, and is attractive or repulsive according to the kind of 

 electricity which.predominates. Affixing to these masses the signs + 

 and - , we may say that the resultant action is proportional to the 

 algebraical sum of the electrical masses contained on the surface, 

 and is repulsive or attractive according to the sign of this sum. 



It is usual to apply the term positive to the electricity developed 

 upon glass when rubbed with resin, and negative to the electricity 

 acquired by the resin. 



7. ELECTRICAL FORCE. The action between two bodies of small 

 dimensions, charged respectively with the masses m and m', and at 

 the distance r from each other, is therefore 



mm' 



f ~r*' 



This expression is positive if the two masses are of the same sign, 

 and the force is then repulsive. In the contrary case it is attractive. 

 If a mass m is in the presence of any electrified bodies whatever, 

 it may be considered that the total action which it experiences is the 



B 2 



