Abstract: A dc commutator motor can be switched off when the motor rotor is short-circuited by the rotor's coming to a halt, by an operating switch (10). The switch is provided in the line connecting the brushes to a radio suppression network in a compact and simple design. The switch is operated by an ac voltage induced when the motor is running, with a frequency corresponding to the motor speed. A sensing winding connected to the operating switch for inducing the operating ac voltage is wound preferably around a radio interference suppression choke of the radio suppression network.

Abstract: A heating device for preheating heating oil flowing through a pipe has a heat conduction body to which a plate-shaped ceramic PTC resistance heating element is connected in heat exchange-relationship. The heating element has a thickness in the range of 0.5 to 2 millimeters, a Curie temperature between 120.degree. and 220.degree. C. and a specific resistance of 430 to 5000 ohm-cm rated at a supply voltage between 110 and 220 volts such that the maximum temperature achieved by the heating element over a range of heat transfer rates is relatively constant and is determined by the resistivity and the Curie temperature of the heating element rather than by the rate of heat transfer. If designed for energization at 220 volts, the heating element has the same thickness and Curie temperature as in the first example described above, but with a specific resistance of 1700 to 20,000 ohm-cm as measured at 220 volts.