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Since the beginning of the 1960s the demister has been a popular accessory in motor vehicles. Its primary purpose is to clear frost, ice and condensation from the rear windscreen. A rear windscreen demister is also known as a rear windscreen heater and is generally operated from the dashboard through the rear demister switch. The rear windscreen demister is an invaluable aid to the driver when weather conditions make visibility through the rear window difficult.
The rear window demister system often consists of a set of resistive electrical conductors on or in the windscreen glass. When the system is activated, by engaging the rear demister switch, the set of conductors are heated up. As they warm the rear windscreen, any frost, ice and/or condensation is also heated up. The ice and frost are thawed whilst the condensation evaporates.
The resistive conductors are either attached to the rear windscreen or embedded within the glass itself during manufacture. The latter type of design is much less prone to damage because the conductors are effectively shielded from external abrasion.
Some demisters will automatically switch off after a certain period, whilst others require the rear demister switch to be used to turn off the system when the windscreen has been effectively cleared.