A system for preventing frost from accumulating on a window of a motor vehicle has a heating element provided on the window, a temperature switch, which is closed when the temperature of window is lower than 0.degree. C., and a humidity switch, which is closed when the humidity of air outside of the window becomes slightly lower than 100% relative humidity. When both the switches are closed, an electric circuit is closed to connect a battery with the heating element, thereby preventing the formation of frost on the window.

A driving circuit for the electromagnetic relay provides a coil of the electromagnetic relay with stepped voltage having a first voltage which is slightly larger than a minimum operating voltage which caused the coil to generate magnetic field capable of closing contacts of the electromagnetic relay and a second voltage higher than the first voltage, in response to a triggering signal to trigger the electromagnetic relay. According to this structure, because the first voltage lower than the second voltage is applied to the coil firstly, the contacts are closed by attracting force of the coil risen gently. As a result, the bouncing of the moving contact can be prevented. In addition, the response time required for the closing of the contacts can be prevented to become long.

A window for buildings, particularly dwelling houses and office buildings, comprising a frame and two or more glass window-panes, of which the window-pane located nearest a room has an electrically conductive layer applied to the side thereof remote from the room. In accordance with the invention there is provided a control circuit (8) for connection to a voltage source located in the frame (2) of the window (1). Also provided is a thermistor (14) which is in contact with the side of the window-pane (3) remote from the room and which is connected to the aforementioned control circuit (8) and arranged to produce or to influence an electric signal corresponding to the temperature of the window pane (3). The control circuit (8) is arranged to control the supply of current to the electrically conductive layer so that the window-pane (3) can be brought to a given temperature set by a potentiometer (16) connected to the control circuit (8).

An improvement is provided for an electrically heated glass panel of the type where a resistive film is disposed on the panel and is connected between and along the length of a pair of spaced bus bars each of which is terminated in a respective voltage input terminal and where one of the bus bars includes a connector section to which the resistive film is not connected extending from the input terminal of the bus bar to a junction point on the bus bar beyond which the resistive film is connected to the bus bar. A sensor conductor is disposed on the glass panel and connected from at or beyond the junction point on the one bus bar to a sensor terminal and extends generally adjacent to the connector section of the bus bar. Prior to applying a high voltage across the input terminals of the bus bars to energize the resistive film when it is desired to heat the glass panel, a low voltage incapable of producing an arc across any break in the connector section of the one bus bar is applied to the input terminal of such bus bar. The power supply is disabled in response to the presence of a predetermined voltage difference between the sensor terminal and the input terminal of the one bus bar indicating the presence of a break in the connector section of the bus bar so as to prevent the development of an arc across the break.

A convertible vehicle having a body with a removable soft top and a separate rear window with an electrically-operable component. The rear window being selectively attachable to and removable from the top. A separable fastener assembly such as a zipper is provided with the top and the rear window so that the electrically-operated component is electrically connected with a vehicle power supply upon attachment of the rear window to the soft top.