Electrical heating devices for plate-like bodies are known in which a resistor is used for generating and transferring heat to the plate-like body. In such devices, it is known to use a resistor having a positive temperature coefficient of resistance. Such devices have become known, for example, for heating of outside rear view mirrors of motor vehicles. In such devices, the resistor having a positive temperature coefficient is used as a self-regulating heating element. A surface of the resistive body is soldered onto a heat distributing plate and an electrical lead is soldered to the plate and connected with the vehicle body and serves as electrical ground. Another surface of the resistive body is connected by soldering to an electrical lead which is connected through a switch to the power supply of the vehicle to complete the heating circuit. Such arrangements are set forth in German patent DE-GM Nos. 7,919,839 and 7,929,548. Such a resistor assembly is very expensive due to the soldered connections which must be of high quality in order to achieve the lowest possible electrical and thermal contact resistance. Also such a resistor assembly is subject to thermal stress cracks in the soldered joints which cause deterioration or failure of the device.
Positive temperature coefficient resistors having a hollow cylinder shaped resistive body with terminals at opposite end faces are known as set forth in German patent application DE-OS No. 1,465,439. Also, the use of leaf springs for contacting a flat circular resistive body inside a casing is known as set forth in German patent application DE-OS No. 2,540,029. In this, the pressing forces are dependent on the dimensions of the casing.
A general object of this invention is to overcome certain disadvantages of the prior art.