The ratio of the square of a length and a length is again a length, so that we have in both cases the capacity expressed as so many cm.
In deducing the conception of capacity we assumed that the conductor has a spherical shape, but obviously if, instead of suspending a sphere and charging it, we had suspended a metallic body of any shape and forced electricity on to it by the frictional machine, it would have acquired some charge proportional to the e.m.f. applied. The body of irregular shape also has capacity, only we may not always be able to calculate it exactly beforehand. It can, however, always be found experimentally. For this purpose we apply a known e.m.f. to charge the body and then discharge it through a special kind of measuring instrument. The instrument indicates the quantity of charge which has passed through it; and from the two measurements, namely, e.m.f. and quantity, we can determine the capacity. For certain shapes the determination of capacity, by mathematical reasoning, is quite easy. One case, namely that of the sphere, either free or in a shell, we have already treated. The case of concentric cylinders, or parallel cylinders, or a cylinder
