a flat normally horizontal electric resistance blanket heater comprising a thin , fragile elongate resistance element established on one surface of and carried by a thin , flexible film of dielectric material , a thin , flat resilient and flexible reinforcing strate of dimensionally stable material retained adjacent the other surface of the film to allow for controlled flexure of the heater and prevent dimensional distortion of the film and element and an envelope of flexible dielectric plastic film encapsulating the film , element and strate . the heater also includes temperature responsive switching means responsive to the temperature of heated portions of the heater and in temperature insulated relationship from a portion of the heater where the element connects with power supply lines and which creates a portion in the heater of greater mass and heat storing capacity .