ELECTRICAL MEASUREMENTS. 273 



existing, then the force exerted on that unit of electricity is 

 the force which we have to deal with in this case. If we 

 can-measure that force we can measure the density. This 

 method, however, is not generally applicable, because we 

 cannot usually place a charge outside a conductor without 

 disturbing the condition of the -conductor itself. The 

 generally applicable method is that by means of the carrier- 

 plane, which I spoke of before. 



The next fundamental magnitude is electrical Potential. 

 This, in the case of electricity, corresponds to temperature 

 in the case of heat, or to the surface level in the case 

 of a liquid. The properties of any charged conductor 

 depend not only on the quantity of electricity it contains, 

 but also upon what we may call in general the quality of 

 the electricity, or the potential. Just so the properties of a 

 body containing heat depend not only on the quantity of 

 heat in the body, but upon the temperature of that heat, or 

 the temperature of the body as we call it. Potential is a 

 magnitude of which we cannot avoid the discussion in 

 speaking of electrical phenomena, for it corresponds to 

 properties which are not expressed by any other term. The 

 potential of a conductor, or rather the potentials of two 

 conductors, determine whether, when they are connected 

 together, there is any passage of electricity from one to the 

 other, and there is no other property which in general 

 determines this. It is easy to convince ourselves that whether 

 electricity passes from one conductor to another or not, does 

 not depend simply on the quantity which either of them 

 contains. Electricity does not always go from a conductor 

 which contains a large quantity to one which contains a small 

 quantity. Suppose this small ball and a larger sphere 

 brought together, the sphere being charged to begin with ; put 

 the ball in contact with the sphere, and then separate them. 

 They are now both charged, and the sphere has got more, 

 electricity than the ball, but if you put them in contact again 

 there is no further discharge of electricity from one to the 

 other. We have unequal quantities, but we have not, there- 

 fore, a passage of electricity from one to the other. Again, it 

 is not a question of the density of the charge, for we have 

 a greater density on this small ball than on the sphere; 

 still when we make the contact no electricity passes. A 

 still more conclusive case of the same kind is .this. Take an 



