Formulæ for the capacitance of a guard-ring sphere-gap are derived by the method due to G. R. Dean (Physical Review1913, vol. 1, p. 316) in which a solution of Laplace's equation is employed, and also by the well-known method of point charges (A. RUSSELL, “ Alternating Currents,” vol. 1, p. 236). Expressions suitable for computation are given for the practical case of equal spheres. A few numerical examples are also given, together with the measured values of the capacitance of a 50-cm guard-ring sphere-gap