A movable-coil transformer consists essentially of an elongated core on which are located primary and secondary coils. Either the primary coil or secondary coil may be movable, while the other one is in a fixed position. Most a-c transformers of this design have a fixed-position secondary coil. The primary coil is normally attached to a lead screw and, as the screw is turned, the coil moves closer to, or further from, the secondary coil.
Figure M-9 shows one form of a movable-coil transformer with coils spread apart for minimum output and a steep slope volt-ampere curve. Figure M-10 shows the coils close together. The volt-ampere curve is indicated at maximum output with less slope than the curve in Figure M-9.
