Xerography (Photocopy Machine)
Applications Of Electrostatics. The copying process is called xerography which is derived from the Greek words xeros and graphos.
Means “dry writing”.
The heart of the machine is the drum which is an aluminum cylinder coated with a layer of selenium.
Aluminum is an excellent conductor whereas selenium is an insulator in the dark and becomes a conductor when exposed to light.
Meaning it is a photoconductor.
As a result, when a positive charge is sprinkled over the selenium it will remain there as long as it remains in the dark.
If the drum is exposed to light, the electrons from aluminum pass through the conducting selenium and neutralize the positive charge.
If the drum is exposed to an image of a document to be copied, the dark and light areas of the document produce corresponding areas on the drum.
The darkest areas retain their positive charge, but light areas become conducting, lose their positive charge and become neutral.
In this way, a positive charge image of the document remains on the selenium surface.
Then a special, dry black powder called toner which has a negative charge is spread over the drum where it sticks over the positive charges.
Applications Of Electrostatics. The toner from the drum is transferred to a sheet of paper on which the document is to be copied, heated pressure rollers then melt the toner into the paper which is also given an excess positive charger to produce the permanent impression of the document.