8 Electrostatics Physical Principles [OH. i 



In this way then, we get the idea of a definite quantity of electrification 

 associated with the brass rod, this quantity being independent of the position 

 of the rod inside the closed vessel of the electroscope. We find, further, that 

 the divergence of the gold-leaves is not only independent of the position of 

 the rod inside the vessel, but is independent of any changes of state which 

 the rod may have experienced between successive insertions in the vessel, 

 provided only that it has not been touched by conducting bodies. We 

 might, for instance, heat the rod, or, if it was sufficiently thin, we might 

 bend it into a different shape, and on replacing it inside the vessel we 

 should find that it produced exactly the same deviation of the gold-leaves 

 as before. We may, then, regard the electrical properties of the rod as being 

 due to a quantity of electricity associated with the rod, this quantity remaining 

 permanently the same, except in so far as the original charge is lessened by 

 contact with conductors, or increased by a fresh supply. 



8. We can regard the electroscope as giving an indication of the magni- 

 tude of a quantity of electricity, two charges being equal when they produce 

 the same divergence of the leaves of the electroscope. 



In the same way we can regard a spring-balance as giving an indication 

 of the magnitude of a weight, two weights being equal when they produce 

 the same extension of the spring. 



The question of the actual quantitative measurement of a quantity of 

 electricity as a multiple of a specified unit has not yet been touched. We 

 can, however, easily devise means for the exact quantitative measurement 

 of electricity in terms of a unit. We can charge a brass rod to any degree 

 we please, and agree that the charge on this rod is to be taken to be the 

 standard unit charge. By rubbing a number of rods until each produces 

 exactly the same divergence of the electroscope as the standard charge, we 

 can prepare a number of unit charges, and we can now say that a charge is 

 equal to n units, if it produces the same deviation of the electroscope as 

 would be produced by n units all inserted in the vessel of the electroscope 

 at once. This method of measuring an electric charge is of course not one 

 that any rational being would apply in practice, but the object of the 

 present explanation is to elucidate the fundamental principles, and not to 

 give an account of practical methods. 



9. Positive and Negative Electricity. Let us suppose that we insert in 

 the vessel of the electroscope the piece of silk on which one of the brass 

 rods has been supposed to have been rubbed in order to produce its unit 

 charge. We shall find that the silk produces a divergence of the leaves of 

 the electroscope, and further that this divergence is exactly equal to that 

 which is produced by inserting the brass rod alone into the vessel of the 

 electroscope. If, however, we insert the brass rod and the silk together into 

 the electroscope, no deviation of the leaves can be detected. 



