Part 1 of the paper deals with an experimental investigation of alternating-current corona discharge in air and other gases, under conditions in which the effects of space charges are negligible. The results are of theoretical and practical interest, an analysis of them resulting in a theory of electrical discharge which forms the basis of Part 2 of the paper.The results of corona discharge for all the gases are found to obey accurately the empirical relationGm=Gℴρ{1+B/√(γρ)}given by Peek, Whitehead, and others. The experimental results for air, andits principal constituents, oxygen, nitrogen, and carbon dioxide, are given in detail, and the constantsG0andBare quoted for a few of the gases which are considered later in the paper in relation to sphere-gap spark discharge.Assuming Russell's theory of sphere-gap spark discharge, it is shown that the condition for this discharge isGm=KGℴρ{1+K'B/√(γρ)}whereG0andBhave the same values as in corona discharge, andK, K, are constants which have the same values for all gases.Part 2 of the paper deals with sparking potentials as a measure of maximum voltages, and is a preliminary account of the work. A new formula is given for uniform-field spark discharge, and this form of discharge is proposed as an alternative to sphere-gap discharge for the measurement of voltages up to 300kV (r.m.s.), the limit of the experiments. Advantages and disadvantages of the two types are also considered. The results of an experimental investigation of uniform-field spark discharge are given, for variations of spacing and gas density.