Electric Carbon Arc Light Television
Experimentation with broadcasting movies by radio and employing electric carbon arc television occurred around 1930, and a small piece of this rare history is well worth touching on here. Although thoughts of transmitting pictures to distance places undoubtedly inspired mankind
since antiquity, the electric carbon arc light may not have brought the reality of big-screen TV before mankind’s eyes until recently. That was when the Roaring Twenties had allowed events like the Derby and the principal race at Epson to be successfully televised through photo-electric cells and received on small TV screens in Great Britain and abroad by the use of neon lamps. However, powerful carbon arc lights were needed to light up larger screens.
An early 20th-century television viewed on a train traveling at 70 miles an hour In "The Wonders of Television," a contemporary article covering the subject, its author, J. J. Denton, A. M. I. E. E., verified the fact and pointed out how this was done. He wrote:
“Most interesting demonstrations are now being shown on a large screen. This is accomplished by making the incoming signals affect or modulate the light of an electric arc lamp. The light is then reflected to its right place on the screen by aid of a correctly speeded rotating mirror drum. In this way, the usual lantern screen may be used, and large audiences can see the projected images of whatever is being broadcast.”
The Honorable Secretary of the Television Society and a Fellow of the British Radio Institution went on to describe in his article the early twentieth-century employment of cathode rays in television receivers, much like they are still used in TV's today, and added:
“Another method of producing the image at the television receiver utilizes polarized light. The usual light lantern is used, its light being made to pass through Iceland Spar, or some other form of polarizer. The light is then plane polarized and has the property of being affected by electrostatic or magnetic fields in such a way that the plane of polarization is turned so that it can pass through a second polarizer placed at right angles to the first, as regards the vibrating planes of the light. Glass under strain exhibits in a beautiful way this effect, as also will many crystals. The effect is much enhanced if the light is made to pass through nitrobenzene, which is stressed by the electrical forces due to the television signals. By this method, a lantern screen can show distant events to quite a large audience.
An early twentieth-century drawing of a five-channel electric carbon arc light television
“In practice, the apparatus consists of the arc lantern, a nicol prism as polarizer, a Kerr cell consisting of a glass cell containing the nitrobenzene and fitted with two or more plates connected to electrodes that receive at a high voltage the incoming television signals. Then, next to this is a small aperture in a plate of metal, through which the light that has been modulated in the Kerr cell passes on to a second nicol prism. When the two nicol prisms are “crossed,” that is, when their crystalline axes are at right angles to each other, almost no light passes. Then, as the received television signal stresses the nitrobenzene, light will pass and illuminate the screen in accordance with the strength of the signal. The result is a very brilliant and nicely detailed image which appears on the screen by aid of the mirror drum, which is revolved so as correctly to reflect on to the screen the light which passes through the second prism. Picture of an early 20th-century big-screen television powered by an arc light in a theater
“This method and the method which directly modulates the flame of the arc light are the most promising for public displays, as in the cinema; or for educational purposes, as in the class rooms of schools.”