Mechanical TV: How it works

The scanning and reproducing discs are similar. Both are mounted on driving motors, and each is punched with a spiral of small holes along the outer edge. The number of holes matches the number of lines of picture definition.

At the transmitter in this mechanical system, the studio is in total darkness. A light emanates from a lamp behind the disc and, projected through the holes set in the spiral on the outer edge, scans the features of the subject's face. The photocell converts these variations in the reflected light into the electric impulses, which, once amplified, can be transmitted by radio waves.

At the receiver, the signal is converted into a sequence of bright flashes by the neon tube. The reproducing disc rotates rapidly in front of this tube, and converts each flash of the lamp into a small element of the image. The rapid speed of the disc makes "persistence of vision" possible for the looker-in.

"Persistence of vision" means that the brain retains an image for one tenth of a second after it is perceived by the eye. The rapid repetition of moving images (in film or television) tricks the brain into perceiving continuous images.

How Black and White Electronic Television Worked circa 1939

The Iconoscope was the camera pick-up tube that was most commonly used in 1939 TV cameras. It was the "eye" of television. Inside the Iconoscope, the image is projected onto a photosensitive plate and scanned by an electron beam, breaking the elements of the image into a series of electrical impulses. These impulses can be transmitted as a radio signal to a TV set.

The receiver contains a picture tube (Kinescope). The Kinescope tube creates another electron beam which varies in intensity while being magnetically deflected very rapidly. When this beam strikes the back of the phosphor coating inside the Kinescope, each spot glows for a split second, until the beam has a chance to return for the next pass.