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An electric resistance heater engageable with work to be heated including a work-engaging heater structure of minor thermal mass per unit area and including an elongate primary heating element arranged to establish high watt density throughout said heater structure, a power supply, a temperature con

An electric resistance heater engageable with work to be heated including a work-engaging heater structure of minor thermal mass per unit area and including an elongate primary heating element arranged to establish high watt density throughout said heater structure, a power supply, a temperature control means between and connected with the primary heating element and the power supply; said temperature control means including a work engaging body of greater thermal mass per unit area than the heater structure and carried by said heater structure in spaced relationship with the primary heating element, a normally closed thermal responsive switching device within the body and operable to open when the temperature of the body is heated by heat conducted from the work to a predetermined operating temperature, an elongate secondary heating element arranged in the body to delivery sufficient heat into the body to normally maintain the temperature thereof at a temperature slightly below said operating temperature whereby little heat need be conducted from the work into the body to raise its temperature to said operating temperature, said elements and device being series connected with the power supply. The device further includes a heat pipe extending between the heater structure adjacent the primary heating element and the body to rapidly and directly conduct heat and to raise the temperature of the body to said operating temperature when the temperature differential between the heater structure and body becomes excessive and before the temperature of the heater exceeds a predetermined maximum operating temperature.

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A heater construction having a portion of minor mass in which an elongate electric resistance primary heating element is arranged and a portion of major mass in which a normally open thermo responsive switching device is arranged and set to open when the temperature of the portion of minor mass reac

A heater construction having a portion of minor mass in which an elongate electric resistance primary heating element is arranged and a portion of major mass in which a normally open thermo responsive switching device is arranged and set to open when the temperature of the portion of minor mass reaches a predetermined, elevated, control temperature and to close when the temperature of the portion of minor mass is below said control temperature, a secondary elongate electric resistance heater element in the portion of greater mass to normally maintain that mass at a mean elevated temperature which is below said control temperature, a power supply circuit connected with the heating and heater elements and in which the switching device is connected to control the supply of power to said elements and an elongate heat conducting heat pipe in and extending between the portions of minor and major mass and conducting heat therebetween whereby the temperature of the portion of greater mass at the switching device is rapidly changed in response to corresponding changes in temperature of the area of minor mass, the portion of minor mass includes a flat, normally horizontal core of dielectric material, said primary heating Lement is an elongate metallic conductor on one surface of and carried by the core and an envelope of dielectric material enclosing the core and including top and bottom laminates with central portions overlying the top and bottom surfaces of the core and edge portions about the perimeter of the core and integrally joined.