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1945 Theatre Catalog, 4th Edition, Page 591 (563)

1945 Theatre Catalog, 4th Edition
1945 Theatre Catalog
1945 Theatre Catalog, 4th Edition, Page 591
Page 591

1945 Theatre Catalog, 4th Edition, Page 591

i'Wliy" of Television in One-Syllable Words

Facts Every Theatre Operator Should Know To Understand Why Television Is Possible

In order that the explanation of how a television system works and the description of the modern apparatus used in television will be clear and comprehensible, we will consider first the theory of electronic devices. ELECTRONIC DEVICES

All of them, naturally, involve the electron, the smallest known quantity of negative electricity. All electrons are alike. All matter is made of electrons. Protons are also found in all matter, the proton being positively charged. Elece trons may be removed from the atoms of certain substances more easily than from others.

All negatively-charged bodies repel each other; so, too, do all positivelycharged bodies. However, negative and positive charges attract each other. In other words, there is a tendency toward a balance of charge, and since a body is said to be charged negatively only because of the absence of protons, or vice versa, it is natural that oppositely charged bodies should attract one another.

Electrons in motion constitute an electric current. Electrons ordinarily move in any direction, but experiments have proved that electrons are similar to tiny magnets and may, therefore, be guided by magnetic fields.

Electrons may be more easily removed from metals in general than from other substances. Electrons are packed in metals so tightly that adjacent atoms share mutually each other's electrons. Since the electrons How about freely in the atoms in all directions, it is possible to impress an electrical pressure or voltage upon the metal and cause electrons to move in a uniformly given direction, thus setting up an electric current in the metal.

Electrons are removed from metals by heat, as in the lamp filament; by light striking certain metals, as in the photoelectric cell; by the impact of projected electrons, and by other methods not important to this discussion.


The three devices which are so important in television are the vacuum amnlifier tube, the photo-electric cell, and the cathode-ray tube. Each utilizes electrons, but in different ways. Without all three devices, television of the modern type would be impossible.

The Vacuum Amplifier Tube

The vacuum amplifier tube is essentially a triode tube. Present-day types have additional electrodes for increased efficiency for certain functions. A triode




Fellow of the Society of Motion Picture Engineers

consists of a filament, a grid, and a plate contained within a vacuum tube. The grid is placed between the other two electrodes. Electrical contacts in the base for each electrode are provided.

The theory of operation of a triode is this: When a current is passed through the fine-wire filament, the filament becomes very hot, causing the electrons therein to be freed. These freed electrons tend to travel in all directions away from the filament. By impressing a relatively high voltage between the filament and the plate, a high positive charge is set up in the plate which strongly attracts the electrons, causing them to flow from the filament to the plate.

The electrons must pass through the grid to reach the plate. If a small voltage be impressed between the filament and the grid, a positive charge is also set up on the grid which increases the attraction to the electron beam and causes it to flow even more quickly and efii

This article by James Frank, Jr., has been condensed from the first three and last of a series of thirteen articles appearing, from October, 191,3, to October, 1944, both inclusive, in the International Projectionist, from which permission to reprint both text and illustrations has been granted.

ciently toward the plate. If, however, the voltage between the filament and grid be reversed, and a negative charge set up on the grid, it will tend to repel the electrons. The how of electrons from the dlament to the plate is thus retarded.

It is evident that if an alternating or fiuctuating current be passed through the grid, it will cause the grid to vary its charge from positive to negative and back in direct proportion to the fluctuating current. This variation in charge will also cause the electron beam to be increased or retarded in direct proportion to the fiuctuating current.

The variation in the speed of the electron beam w1ll vary the current flowing in the plate. This latter current may be made to be of much greater intensity than the current being passed through the grid by impressing a high voltage between the filament and the plate. The electrOn beam simply increases and decreases this current. Thus the plate current is directly proportional to the grid current in fluctuation and of larger intensity. In this manner, the triode has been made to amplify the current.

A vacuum amplified tube such as a triode can be made to do several things, depending on the type of circuit in which it is employed. It can be made to act as a relay, an amplifier tube, a rectifier for changing alternating current to direct, and an inverter changing direct current to alternating current.

Photo-Electric Cell

There are certain kinds of metals which emit electrons if exposed to visible light. It has been shown that all metals will

FIGURE l.-A photo-electric cell is a vacuum tube in which a cesium-coated cathode (negative electrode) and an anode (positIVe electrode) are placed. The cathode is generally semi-cylindrical in shape, with the cesium coating on the concave surface. The anode is generally placed at the center of the cathode's curvature. The degree of sensitivity of these cells may, in many cases, be very much increased by filling the tube with certain inert gases.




1945 Theatre Catalog, 4th Edition, Page 591