A three-terminal device (1, 21, 31) which can operate as an electrical switch, controlled by a third terminal (3, 24, 36/36A) that controls switching the device (1, 21, 31) from a closed circuit (conducting) state to an open circuit (insulating) state. Polymer and/or ceramic materials are loaded with different conductive materials, forming a device (1, 21, 31) having various electrical conductivities depending on local temperature. In preferred embodiments, the device (1, 21, 31) exhibits a non-linear increase or decrease in resistance versus temperature. Various embodiments of the three-terminal devices (1, 21, 31) are disclosed. The third terminal (3, 24, 36/36A) may be a non-electrically conductive biasing element such as a thermal transfer rod or tube containing a variable temperature fluid.