In 1873, the photoconductive properties of the element selenium were discovered, the fact that selenium's electrical conduction varied with the amount of illumination it received. Paul Nipkow created a rotating scanning disk camera called the Nipkow disk, a device for picture analyzation that consisted of a rapidly rotating disk placed between a scene and a light sensitive selenium element. The image had only 18 lines of resoution.
Nipkow DiskAccording to R. J. Reiman author of Who Invented Television: The Nipkow disk was a rotating disk with holes arranged in a spiral around its edge. Light passing through the holes as the disk rotated produced a rectangular scanning pattern or raster which could be used to either generate an electrical signal from the scene for transmitting or to produce an image from the signal at the receiver. As the disk rotated, the image was scanned by the perforations in the disk, and light from different portions of it passed to a selenium photocell. The number of scanned lines was equal to the number of perforations and each rotation of the disk produced a television frame. In the receiver, the brightness of the light source would be varied by the signal voltage. Again, the light passed through a synchronously rotating perforated disk and formed a raster on the projection screen. Mechanical viewers had the serious limitation of resolution and brightness.
No one is sure if Paul Nipkow actually built a working prototype of his television system. It would take the development of the amplification tube in 1907 before the Nipkow Disk could become practical. All mechanical television systems were outmoded in 1934 by electronic television systems.