The construction and properties of certain superconductive amplifiers in which the resistance of a gate element is controlled by the electromagnetic field of a signal coil are described in this paper. Vacuum-deposited films of tin, which have narrow resistive transition widths, have been used as the gate elements, and the field has been applied perpendicularly to the film surface. The parameters of small-signal analysis are determined, and the influence of temperature, bias magnetic field, and zero-signal gate current upon power gains are shown. The limitations and merits of the device in its present form are discussed. © 1965, IEEE. All rights reserved.