Abstract: An a.c. Resonance Transformer for operating at a predetermined frequency. The transformer comprises a resonant .pi.-type or T-type stage. The .pi.-type stage includes three series connected four-terminal networks. The first of the networks and the third of the networks includes a shunt reactor of a first type. The second of the networks includes two series connected second type reactors and a first type reactor shunt connected to the junction between the second type reactors. The reactors in the second of the networks are sized so that when an input signal of a predetermined frequency is applied to the input of the first of the networks, a 180.degree. phase shift is provided between the phase angle of the input and output voltage vectors of the second of the networks and so that a zero input impedance is provided when the output is short circuited.