The type that’s on a old calculator. Black or monochrome? Some bars would go missing some times but you can fix it by physically manipulating the screen or casing or hitting it.
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Lcd screens work by using electricity to control liquid crystals to sort of “bend” the light in a way makes it be blocked or pass though a polarizer filter
The screens are wired like a grid so you only need one wire for each line and one for each row instead of one for each pixel. They turn on only one row (or column) at a time with the correct pixels and then go to the next, very fast so it looks static. This means that if the wire for a column goes out only that line will not work.
By touching the screen and bending it a bit you are making a better contact on the lines that failed and it starts going again
The old calculator screens that you describe are liquid crystal displays, the same kind of technology used in many modern displays. Their basic operating principle is that they contain a sort of viscous liquid within which are embedded many tiny crystals. Those crystals are special in that they block the passage of light when oriented in one direction while allowing it to pass through in a different orientation, and they can be manipulated with electric fields.
By mixing them into this viscous liquid and passing current through conductive layers on the front and back of the container, the crystals can be caused to all orient themselves one way or another, blocking or allowing light through the liquid. Form those containers into the shape of pieces of numbers and you can make the calculator display. Calculators usually just have an off-white reflective back so you can see the contrast of the number sections against it, while other displays may have a white light shining from the back to form more complex color images.
As for why the screens break in certain ways, this is often due to the conductive layers which carry the electricity to the liquid crystal cells. Those layers can become de-laminated and make poor electrical contact between the electronics and the cells meaning the screen doesn’t work as desired. By physically manipulating the screen or hitting it that connection that just barely broke might be returned to a connection (although tenuous).
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