sphere was in a large room or in the open. Comparing now the two cases, namely, the sphere in the open and the sphere closely surrounded by a metallic envelope, it will be seen that to get the same charge on the spheres is not equally easy. The sphere hanging free requires the application of a much larger e.m.f. than the sphere within an envelope, or, to put it another way, the sphere with an envelope will, under the application of the same e.m.f., acquire a much greater charge than the sphere hanging free in space. The capacity of the sphere for taking a charge has been increased. This reasoning leads us to the conception of capacity as a property of the configuration of metallic bodies. We define capacity as the ratio of charge divided by e.m.f. Using the symbol for capacity, the definition mathematically expressed is
and
Since we found previously that , it follows that the capacity of a sphere in the open is given by the length of its radius expressed in cm. For the sphere with its envelope the capacity is
