PURPOSE: To ensure safety and eliminate the danger of overheat and electric leak by constituting an electric heating element on the rear side of a thermally discoloring layer from a thermistor wherein temperature coefficient of electric resistance shows positive value in a specific temperature range, specific volume resistivity is provided, and heat saturation temperature is self-controlled to a specific temperature. ;CONSTITUTION: A thermally discoloring layer 3 is formed on the surface of an electric heating element 2 itself or on the surface of a heat transfer member 4 disposed close to the element 2 or a base material 5 in contact with the member 4 to change the color of the layer 3 by energizing the element 2, wherein the element 2 is constituted from a thermistor wherein temperature coefficient of electric resistance shows positive value in the range of 25-65°C, volume resistivity (ρ25) at 25°C is 9.8×10-3-2.97×105Ω.cm, the ratio of volume resistivity (ρ65/ρ25) between 25°C and 65°C is within the range of 5≤(ρ65/ρ25)≤200, and heat is generated by application of voltage of 0.8-40V and heat saturation temperature is self-controlled to any temperature within the range of 25°C-65°C.;COPYRIGHT: (C)1994,JPO&Japio
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