perature of said heat sink, the thermal resistance of said variable thermal resistance increasing when the temperature of the heat sink decreases and decreasing when the temperature of the heat sink increases.
2. The invention as defined in claim 1 wherein the variable thermal resistance varies in a manner to maintain the rate of dissipation of heat from said controlled body substantially -constant whereby the operating temperature of said controlled body is maintained substantially constant.
3. In combination, a controlled body including a source of heat, generated at substantially constant rate, a heat sink having an uncontrolled temperature which is normally lower than the permissible operating temperature of the controlled body, and variable thermal resistance means thermally connecting said controlled body to said heat sink for dissipating the heat from the controlled body to the heat sink at a controlled rate, said variable thermal resistance means being controlled by the temperature of said heat sink, said variable thermal resistance means comprising first and second thermally conducting walls engaging said controlled body and heat sink respectively and fixedly spaced apart by a thermal insulating wall in a manner to define an enclosed cavity therebetween, and a temperature responsive element within said -cavity engaging said second wall and cooperating with said first wall to define a variable gap therebetween, said temperature responsive element varying the width of said gap in response to temperature changes in said heat sink to control the rate at which heat is dissipated across the gap.
4. The invention as defined in claim 3 wherein the width of said variable gap decreases as the temperature of the said heat sink increases and increases as the temperature of said heat sink decreases.
5. In combination, a controlled body including a source of heat, generated at substantially constant rate, a heat sink having an uncontrolled temperature which is normally lower than the permissible operating temperature of the controlled body, and variable thermal resistance means thermally connecting said controlled body to said heat sink for dissipating the heat from the controlled body to the heat sink at a controlled rate, said variable thermal resistance means being controlled by the temperature of said heat sink, said variable thermal resistance means comprising bimetallic means thermally connected between said heat sink and controlled body and engaging at least one of the two with a variable contact area, said contact area increasing as the temperature of the said heat sink increases and decreasing as the temperature of the said heat sink decreases.
6. A variable thermal resistance device comprising first and second walls of thermally conductive material fixedly spaced apart by a side wall of thermally insulative material to -define an enclosed cavity, an expandable bellows positioned within said cavity having one end thereof thermally connected to said first wall with the other end thereof closed off by an end plate of thermally conductive material, said end plate being positioned adjacent to said second wall to define a variable gap therebetween, and a thermally expansive medium filling said expandable bellows to vary the width of said gap in response to temperature changes experienced by said first wall.
7. In combination, a controlled body including a source of heat, generated at a substantially constant rate, a heat sink of uncontrolled temperature normally lower than the permissible operating temperature in the controlled body, and a variable thermal resistance device comprising a member made of a thermal conducting material connected to said heat sink and a bimetallic temperature responsive element connected to said member and engaging the surface of said controlled body with a variable contact area which varies directly with heat sink temperature.
8. A temperature control system comprising: a controlled body; heater means operable to supply heat to said body; a heat sink of uncontrolled temperature normally lower than that of said body; a variable thermal resistant device, inversely responsive to the temperature of said sink, interposed in thermally conductive relation between said body and heat sink; said variable resistance device including a temperature responsive member in a continuous heat conductive relation with said heat sink; and switching means effective upon the attainment of a condition of maximum thermal resistance by said device to render operative said heater means.
