ELECTRICAL MEASUREMENTS, 267 



would not be available for comparisons except in experiments 

 made very quickly one after the other. Again, the Electro- 

 phorus:, every time we lift the cover and take a spark from 

 it, gives us the same quantity of electricity ; the cover always 

 comes up with the same quantity, so that we might charge a 

 conductor from the cover of the electrophorus by giving it 

 several sparks one after the other, and we might estimate 

 the charge by countiug the number of sparks which were 

 given. That at first sight seems, as far as it goes, a perfectly 

 satisfactory method of measurement ; but, on consideration, 

 you will see it is not quite so good as it appears, because 

 although the electrophorus possesses each time the same 

 quantity of electricity, it does not give the same quantity 

 each time to the conductor with which we put it into contact. 

 To take a definite example, suppose that when I let the 

 electrophorus touch this sphere it gives up half its electricity 

 to the sphere : then I charge it again and I put it in contact 

 again, it does not give up so much as before. To begin with> 

 it came in contact with an uncharged sphere, but now it 

 comes in contact with a sphere which has already half as much 

 electricity as the electrophorus itself, so that at each repeti- 

 tion of the process we give a smaller and smaller quantity to 

 the sphere, and you will very easily see that on repeated 

 contacts the quantities of electricity imparted to a good con- 

 ductor diminish according to the terms of a decreasing 

 geometrical series. It is clear that the more nearly the 

 potential of the sphere agrees with the potential of the electro- 

 phorus, the less is the quantity of electricity which passes 

 from one to the other ; and the same thing applies in the case 

 of an electrical machine. If we connect a machine with a 

 Leyden jar or with a number of Leyden jars, one turn of the 

 handle will cause a certain quantity of electricity to pass into 

 the jar ; another turn will cause almost the same quantity 

 to pass in, but you know that after a time the jar gets as 

 strong a charge as the machine we employ can give to it, and 

 then we may go on turning the machine as much as we like, 

 and the charge in the jar does not increase. Hence the 

 proportionality between the charge in the Leyden battery and 

 the number of turns of the machine only holds good at first. 

 If we go on turning we come to a time when we do not increase 

 the charge at all. The quantity produced is however exactly 

 proportional to the number of turns, if the machine remains 



