"My father hated radio; he couldn't wait for television to be invented so he could hate that too."
Peter De Vries, writer
Selenium has a lower electrical resistance when it is soaked in bright light than it does when it is in darkness. This means that by altering the light shone onto a selenium cell, the amount of electricity that can pass through it can be changed. This, combined with the knowledge that all we see is made up of different shades of light and dark, allowed German engineer Paul Nipkow (1860-1940) to come up with a way to convert pictures into electrical signals.
Nipkow's system worked by using a rotating disk that has a spiraled series of holes punched in it. When the disk turns, the moving holes break up the image into a series of varying light signals. When light passes through the holes onto the selenium cells, the selenium reacts. The electrical signal created by passing a current through these selenium cells can then travel by wire to power a lamp set up elsewhere. If a second disk rotating in sync with the first is placed in front of this lamp, the light signals will match up with the holes in the second disk, creating a replication of the original image. If this is done at high speed, the eye will no longer be able to recognize that the light signals are being emitted in quick succession and will instead see one whole image. This process of breaking up an image into small dots, or pixels, is the basic principle on which television is based.