We have all wondered at least once why the rear window from the cars has those horizontal lines orange or black and, above all, because the windshield doesn’t have one. These lines are actually electrical wires which, when crossed by current, heat up thus allowing it defrost of the rear window and improving visibility for the driver.
What are black or orange lines on the rear window of cars and how do they work
The lines behind the glass of our machine are used to heat the window and are useful if we want to defrost the glass or keep it from fogging up while driving, to maintain a good view. A is generated within the lines electrical resistance. Electrical resistance is the ability of a material to oppose the passage of electric current. At the basis of the functioning of the horizontal lines there is therefore theJoule effectaccording to which an electrical conductor produces heat when a current flows through it. Those horizontal lines are actually small cables, very thin wires made of metal and resin that are applied to the rear window with an adhesive method or are actually inserted into the glass.

Electric current is sent from the car battery to these wires and a resistance is created which it generates heat. The thread-like electrical resistor is made of a high-resistivity material, such as tungsten or nickel-chromium. These coated threads are applied to the rear window via screen printing (a printing method) or, more commonly, they are encapsulated between the two layers of glass that often make up the rear window.
When the current reaches the wires the glass heats up slightly, just enough to help clear it of condensation, snow or ice. The lines may also increase where the windshield wipers are located, because in ice and snow, the heat helps clear them faster.

We can activate the defrost function through a button on the dashboard with the rectangle-shaped symbol and the braids going up. That is precisely the symbol of heated rear windowi.e. the anti-fogging and glass defrosting device.
The function should turn off on its own after about 15 minutes or the light will indicate that it is still active.
(PS: the other similar symbol is the windshield one)
Why does the rear window have lines but the windshield doesn’t?
Many people think that the lines are not there because they could obscure the view while driving. In reality, their quantity is appropriately controlled: the opaque circuits must not exceed 5% of the field of vision of the rear-view mirror and the lines must not be wider than 1 mm and no closer than 25.4 mm to each other (SAE J216 ).
These numbers come from precise studies conducted on visibility driving. Already in 1991 a study conducted by Southall and Burnand examined the influence of electrically heated windscreens on the driver’s ability to see. After various tests, of the 20 subjects examined, only 6 declared having noticed the wires embedded in the windshield and only one reported visibility problems. So that’s not the problem.
The reason why the windshield does not have lines is because inside the car, at the front glass, there are lines air vents on the dashboard. The air flow that arrives from there remains the fastest way to defrost car windows. On the back, however, there are black lines.