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corresponds to an ellipsoidal equilibrium surface; the other to a hyperbolic surface whose lines of intersection with planes passing through the axis of $x$ are the lines of electric force. The lines of electric force for a charged sphere in motion are not radial but form a series of hyperbolas.

Figs. 2 and 3 show the forms of the equilibrium surfaces and of the lines of electric force, for a line and a disk respectively, when $\alpha ={\tfrac {1}{4}}$ so that $u/v=.866$.

In fig. 2 the curve marked $\lambda ={\tfrac {4}{3}}$ is a circle, a section of the sphere of which the