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1925 - revolutionized scientific thought, a device which utilizes the mechanical power of light rays and opens the way into an entirely new field of development


E&MP 94.009

Photo Electric Cells

October 21, 1925

Based upon a principle which revolutionized scientific thought, a device is on exhibition at the New York Electrical Show, Grand Central Palace, which utilizes the mechanical power of light rays and opens the way into an entirely new field of development.

The device exhibited is very simple.

It consists of a radio tube of a highly special character, and ordinary door bell circuit, and an automobile headlight.

The light from the headlight is concentrated upon the tube and under these circumstances an electric current flows through the plate (or telephone) circuit of the tube which holds open a switch in the bell circuit.

With, however, the slightest interference in the intensity of the illumination of the tube, such as is caused by the shadow formed by a whiff of cigarette smoke, the current in the tube's plate circuit decreases, the switch closes, and the bell rings. The discovery of the principle on which this device is based was one of the truly startling scientific events, since it altered prevailing ideas on the structure of matter and upset the classic wave theory of light.

Electricity Forced From Metal by Light

It was found that when a ray of light falls upon certain metals, especially those of the so-called alkali group such as sodium and potassium, a stream of electrons is forced out.

This electron stream, as everyone with a knowledge of radio engineering knows, constitutes an electric current.

The current thus produced is very small, but by suitable amplifying apparatus its presence can be demonstrated through various laboratory experiments.

This "photo-electric" effect, because of the minuteness of the current, has heretofore remained a scientific curiosity, but through the invention of V. K. Zworykin of the Westinghouse Electric Research Laboratories, East Pittsburgh, it has now become possible to apply it practically.

Mr. Zworykin's invention consists in adding a photo-electric element to what is practically a standard radio tube.

The tube is elongated, and the electron-emitting metal, such as potassium, is coated on the inside of the bulb.

When light falls upon the sensitive coating, a flow of electrons is established, and this current is utilized to control the plate (or telephone) circuit of the tube.

Current Controlled by Light Intensity

By suitably selecting the electrical characteristics of the tube and the accompanying A, B and C batteries, Mr. Zworykin produces a device in which no current will flow through the tube's plate circuit when the sensitive metal film is in darkness, but as soon as light strikes this film the electrical barrier preventing the flow of current is removed and the current will flow through the plate circuit of the tube in proportion to the intensity of the light falling upon it.

In daylight or under an electric light the current flow thus produced is sufficient to operate a standard telegraph relay, which in turn can be arranged to control electrical or other apparatus to any desired extent.

By reversing connections the effect of the operation of the tube can be reversed, so that it will produce current in darkness, but will prevent current flow when fully illuminated.

The device as exhibited is useful as a fire alarm in such places as the holds of ships, automatic unattended electrical stations, store rooms of buildings and other places.

At the least suggestion of smoke, due to overheating from any cause, an alarm can be sent out by both radio and wire to any desired number of stations.

Reacts in One One-Hundred-Thousandth of Second

In addition, this same device can be used in connection with variations of light for any purpose, such as the exact scientific matching of colors, the detection of flaws in tinplate and textiles, the turning on of street lights at twilight and extinguishing them at dawn, and for innumerable other applications.

The reaction of the Zworykin tube to light variations is extraordinarily rapid, being on the order of one one-hundred thousandth of a second, and since, when incorporated in a suitable circuit, it will send out radio impulses of any desired frequency in direct proportion to the amount of light falling upon it, it opens the way for developments into an entirely new field.

Scientists who are familiar with the tube believe that it will form the practical basis by means of which the transmission of actual scenes by moving pictures by radio will eventually be possible.

(Supplementary data--Mr. Zworykin's technical description of his tube. Wiring diagram of smoke detector. Close-up view of Zworykin's tube. Photograph of Mr. Zworykin with his tube.)

Original Caption by Science Service
© Westinghouse Electric Research Laboratories

National Museum of American History


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