MAKE has a round-up of dead media tech, including a bit by Trevor Blake about mechanical television:
Television achieves the illusion of motion in a similar but unique fashion. Rather than refresh the entire image at once, as film does with each cell that passes in front of the projector’s light, television refreshes an image one line at a time in a scanning process. Within the cathode ray tube, an electron gun scans a single line of an image from one side to the other, then scans the line underneath it, until it has scanned an entire image.
The Nipkow disk is an earlier, mechanical means of achieving the same side-to-side, top-to-bottom scan process. It consists of a disk that rotates on its axis. A series of evenly spaced, uniformly sized holes are cut into the disk, spiraling in toward the center. The disk is housed in a box with a small viewing window: the outermost hole of the disk will form the outermost scan line visible in the viewing window, and each additional hole will form additional scan lines.
The story of mechanical television is an interesting one. Be sure to also check out Wikipedia’s entry as well.