7-11] The Fundamental Conceptions of Electrostatics 9 



Again, let us suppose that we charge a brass rod A with a charge which 

 the divergence of the leaves shews to be n units. Let us rub a second brass 

 rod B with a piece of silk C until it has a charge, as indicated by the electro- 

 scope, of m units, in being smaller than n. If we insert the two brass rods 

 together, the electroscope will, as already explained, give a divergence corre- 

 sponding to n + m units. If, however, we insert the rod A and the silk C 

 together, the deviation will be found to correspond to n m units. 



In this way it is found that a charge of electricity must be supposed to 

 have sign as well as magnitude. As a matter of convention, we agree to 

 speak of the m units of charge on the silk as m positive units, or more briefly 

 as a charge + m, while we speak of the charge on the brass as m negative 

 units, or a charge m. 



10. Generation of Electricity. It is found to be a general law that, on 

 rubbing two bodies which are initially uncharged, equal quantities of positive 

 and negative electricity are produced on the two bodies, so that the total 

 charge generated, measured algebraically, is nil. 



We have seen that the electroscope does not determine the sign of the 

 charge placed inside the closed vessel, but only its magnitude. We can, 

 however, determine both the sign and magnitude by two observations. Let 

 us first insert the charged body alone into the vessel. Then if the divergence 

 of the leaves corresponds to m units, we know that the charge is either + m 

 or m, and if we now insert the body in company with another charged body, 

 of which the charge is known to be 4- n t then the charge we are attempting 

 to measure will be -f m or m according as the divergence of the leaves 

 indicates n + m or n^m units. With more elaborate instruments to be 

 described later (electrometers) it is possible to determine both the magnitude 

 and sign of a charge by one observation. 



11. If we had rubbed a rod of glass, instead of one of brass, on the silk, 

 we should have found that the silk had a negative charge, and the glass of 

 course an equal positive charge. It therefore appears that the sign of the 

 charge produced on a body by friction depends not only on the nature of the 

 body itself, but also on the nature of the body with which it has been 

 rubbed. 



The following is found to be a general law : If rubbing a substance A on 

 a second substance B charges A positively and B negatively, and if rubbing 

 the substance .B on a third substance C charges B positively and C negatively, 

 then rubbing the substance A on the substance C will charge A positively 

 and C negatively. 



It is therefore possible to arrange any number of substances in a list such 

 that a substance is charged with positive or negative electricity when rubbed 



