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The Nipkow disk was a device which its inventor, Paul Nipkow, thought could be used to transmit pictures by wire. The disk had a spiral of holes cut into it. These holes were positioned so that they could scan every part of an image in turn as the disk spun around. The light coming from each point would then be turned into in electrical current.

This electrical signal would light up a second light at the other end of the wire. The second light would flicker because the amount of current it received would depend on the brightness of the image being scanned. The light from this light bulb passing through a second disk spinning at the same speed, would then project the picture onto a screen.

Nipkow's Mechanical Television System, 1884

In 1884, German inventor Paul Nipkow designed a way to transmit images electrically using a pair of spinning metal disks. He punched holes in a spiral pattern along these "Nipkow disks," one of which was housed in a transmitter and the other in a receiver.

The first disk "scanned" an object as light shone through its holes onto selenium, changing the chemical element's resistance and thus its electrical current. The current would move over a wire, causing a lamp in the receiver to flicker. Anyone looking at the lamp through the second spinning disk saw the original image revealed. Nipkow's development led to the first public demonstration of television in 1926.

Source: http://www.discovery.com/

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