276 LECTURES TO SCIENCE TEACHERS. 



electricity into it if we double its potential. The electrical 

 capacity of the conductor is the quantity of electricity required 

 to produce unit change of potential. If the potential of the 

 conductor be zero before we give to it the charge, then the 

 quantity required to charge it to unit potential measures its 

 capacity. In general terms, if C stands for the capacity of a 

 conductor, and if the quantity Q of electricity given to that 

 conductor increases its potential by the amount V, then C = 



^., and ^ would be the quantity required to increase its 



potential by unity. Or, to state the relation in another way, 

 the total quantity of electricity a conductor contains equals 

 the product of its capacity into the potential of the charge, 

 or Q = CY, just as the quantity of gas in a vessel might be 

 measured by the product of the volume into the pressure. 



From this relation you see that if we know the quantity and 

 potential or their ratio, we know the capacity ; if we know 

 the capacity and potential, we know the quantity ; or again 

 if we know the capacity and the quantity we can determine 

 the potential, so that the measurement of these three things 

 cannot be separated from each other. The quantity of 

 electricity in a conductor can be measured by one or other 

 of the processes I have indicated, and its potential can 

 be measured by electrometers, and the ratio of these measures 

 the capacity. That is the general method of absolute 

 measurement. When we have merely to compare different 

 quantities, we may employ methods analogous to those 

 employed for measuring capacities for heat. For instance, to 

 determine the specific heat, or capacity for heat, of a substance 

 we put this substance at a known temperature into a known 

 mass of water at another known temperature, and observe 

 the changes of temperature of the two. The temperature of 

 the immersed body falls and that of the water rises, and they 

 come to equilibrium at some intermediate point. The same 

 quantity of heat is lost by one that is received by the other, 

 and the capacities for heat of the immersed body and of 

 the water are inversely proportional to the changes of 

 temperature which they respectively undergo. So in com- 

 paring the capacities of two electrical conductors : suppose 

 we have one of them charged to the potential V, and the 

 other at zero or uncharged, then, when we allow them to 

 come into contact, they both assume a common potential, 



