ELECTRICAL MEASUREMENTS. 277 



say v, intermediate between these two. One body has lost 

 the same quantity of electricity that the other has received. 

 The potential in. one case has fallen through V - v t and in 

 the other it has risen through + v and the capacities are 

 in the inverse ratio of the changes of potential caused by the 

 loss or gain of the same quantity of electricity ; so that if 

 the first body has the capacity C, and the second body the 

 capacity c, the capacities are to each other in the inverse 



ratio of the changes of potential, or ^ ^-^-. That is the 



< v 



principle of a great number of methods of comparing the 



capacities of different conductors. There is not enough time 

 to enter on the experiment in detail, or even on the principle 

 of other methods which might be given. If we have 

 the capacity of any one conductor measured in absolute 

 measure, the capacity of others can also be determined in 

 absolute measure by some process of comparison such as I 

 have indicated ; if we know the absolute value of one 

 thing, and can compare others with it, we know their 

 absolute value also. Upon the table are standard con- 

 densers or accumulators, as they are better called. They are 

 conductors of carefully ascertained capacity. They are of 

 different forms and different values. One is a very large one 

 formed practically by a combination of small ones. Here 

 again is an instrument devised by Sir William Thomson 

 which is a conductor of variable capacity ; its capacity can be 

 diminished by drawing out the cylinder, and increased by push- 

 ing it in, so that it can be adjusted so as to have the same 

 capacity as any particular conductor we have to measure and 

 then we can read off on the scale the capacity of the other one. 



The Energy of charged conductors is proportional both to 

 the quantity and to the potential ; the absolute energy of a 

 charge of electricity is equal to half the product of the charge 

 into the potential, or ^ QV = E. That can be proved by an 

 apparatus like this. You discharge the electricity through a 

 fine wire stretched across the bulb of an air thermometer and 

 observe the amount of heat. This gives us a quantity which 

 is proportional to the energy of the charge. 



I must be content with merely indicating these points ; and 

 in the lecture to follow on Wednesday next I shall have to 

 speak of some of the chief measurements connected with 

 electrical currents or dynamical electricity. 



