The push-pull mutator invertor is a d.c. arrangement with grid-controlled mercury-arc mutators operating alternately and causing an alternating current to flow in the load circuit. In the case of resistance load practically any shape of alternating voltage wave-form between rectangular form and triangular form can be produced. The shape of the alternating voltage waveform depends upon the capacitance of the invertor capacitor, a relatively small capacitance giving the alternating voltage a rectangular form and a large value making it triangular. The ripple factor of the direct current is governed by the smoothing effect of the inductance in the d.c. circuit, a small inductance causing the direct current to flow intermittently. The degree of regulation of alternating voltage at any change of load is determined by the capacitance of the invertor capacitor. A large capacitance may produce such a large voltage regulation that the curve of power output plotted against load describes a V form, the power output being increased when the load is reduced. The invertor stops working when overloaded.Calculations are carried out for approximate circuit conditions. The results of experiment agree satisfactorily with the theory.