The use of saturable chokes to provide an automatically variable impedance by static means is now well known. The paper is mainly concerned with a method of determining the size and other design details for any given application in as general a way as possible, by basing it only on a knowledge of the load current and voltage magnitude and ratio to be produced.For reasons set out, empirical methods are preferred for information leading to the available impedance ratio from a given core material. The required impedance ratio is deduced from the application in hand. These two aspects are then brought together to give the design details.The effect on the size of the chokes of their two positions relative to the transformer is considered, and the better arrangement for given application is indicated. Highly sensitive devices with a simple control are specially considered.The paper concludes by indicating an approach to the economic consideration of the use of saturable chokes.