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1945 Theatre Catalog, 4th Edition, Page 592 (564)

1945 Theatre Catalog, 4th Edition
1945 Theatre Catalog
1945 Theatre Catalog, 4th Edition, Page 592
Page 592

1945 Theatre Catalog, 4th Edition, Page 592

emit electrons if exposed to certain types of light, many invisible, but with varying degrees of sensitivity. The metals of the alkaliselithium, sodium, potassium, rubidium, selenium, and cesiumare very sensitive to light. When light is directed at thin surfaces of these metals, photo-electrons are emitted, the quantity and speed of which depends on the intensity of the light.

A photo-electric cell (Figure 1) is a vacuum tube in which a cesium-coated cathode (negative electrode) and an anode (positive electrode) is placed. The cathode is generally semi-cylindrical in shape with its cesium coating on the concave surface. The anode is generally placed at the center 0f the tube and at the center of the curvature of the cathode. A small voltage is impressed between the cathode and the anode, so that the anode becomes positively charged and attracts photo-electrons. Then, as light is directed at the cathode or photo-sensitive surface, photo-electrons are caused to be emitted in proportion to the intensity of the light. These photo-electrons How in a beam toward the anode and cause a fiuctuating current in the cathode-to-anode circuit in proportions to the variation in the light intensity. The electric current produced is very small in intensity. The sensitivity of these cells may, in many cases, be very much increased by filling the tube with certain inert gases.

Cathode-Ray Tube

A cathode-ray tube is one in which a stream or beam of electrons is created by a heated cathode or filament. This beam is caused to move with great rapidity along the tube by one or two anodes. The beam may also easily be controlled by both electromagnetic and electrostatic fields so that it can be both focused and deiiected. Certain types of cathode-ray tubes may also include a grid similar to that in a triode, whereby the electron beam is controlled in intensity by impressing the fluctuating current on the grid.


In order better to appreciate television communication, let us first view briefly the simpler form of radio communication.

The function of the radio transmitter is to excite or effect the medium or space between the transmitter and receiver antennas by means of electric currents which in some way represent the intelligence being transmitted. The function of the receiver is to detect these disturbances, translate them back into electric currents like the ones at the transmitter and then convert the currents to the form of intelligence which they represent at the transmitter.

The transmitter consists essentially of three parts: (1) the generator, used to generate or create the particular form of electric currents required; (2) the modulator, the device that controls or governs the generated currents in such a way as to represent the form of intelligence that is being transmitted, and (3) the antenna, radiating the modulated currents into space.

The Generator

The generator in a radio transmitter is similar to that used for developing alternating current for electric light and power, except that the current generated is of considerably higher frequency. We are accustomed to thinking of electric power in terms of frequency of 60 cycles a second. That means that the current has 60 pulses of fiow in each direction a second. The currents that are used in the antenna, however, have a frequency of thousands and millions of cycles a second.

The ordinary radio-broadcast frequency band in the United States is from 550,000 cycles (550 kilocycles) to 1,600,000 cycles (1,600 kilocycles) a second. Television employs still higher frequencies, of the order of 50,000,000 cycles, and laboratories are even experimenting with frequencies of more than 1,000,000,000 cycles a second.

FIGURE 2.-The typical connection scheme of a radio telegraph Iransmiiiing station is diagrammed here. No cur rent will flow in the anienna except when the key is closed. By varying the length of time that the key is closed, dots and dashes are transmitted. In radio telephony, the high-frequency electric current is fed to the

antenna continuously, but if is varied in intensity, or strength, in accordance with the sounds to be iransmmed.



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Function of the Modulator

The modulator is that part of the syso tem which controls the high-frequency alternating currents in the antenna and so modifies them that they represent intelligence.

In the case of. radio telephony, the high-frequency electric current is fed to the antenna continuously, but it is varied in intensity or strength in accordance with the sounds to be transmitted. To do this a microphone is employed, because it is able to convert, the sound waves which strike it into directly proportional electric currents which vary in exact representation of the sounds. The minute currents from the microphone, after being suitably amplified, are then used to control, or modulate, the generated currents fed to the antenna by the generator, thus causing the antenna currents to be representative of the originating sounds. To accomplish this, the amplified alternating currents created by the microphone are added to and subtracted from the generated currents.

Figure 2 shows the typical connection scheme of a radio telegraph transmitting station. It is evident that no current will fiow in the antenna except when the key is closed. By varying the length of time that the key is kept closed, dots and dashes are transmitted. Figure 3 shows in a similar manner a radio telephone transmitting station. Of course, the microphone is not directly in the antenna circuit in practice, but is coupled to it through amplifiers. Here the current may be seen flowing in the antenna, being modulated by the microphone. Figure 4 illustrates how an alternating current from a microphone modulates the constant high frequency antenna current. It may be noted that with the microphone inoperative, the antenna current is of constant intensity, the wave forms being of uniform height. The height of these wave forms is increased and decreased by the microphone current.

The reception of the radio waves involves the generating of electric currents in the receiving antenna when it is struck by the traveling waves of varying strength. These currents, after being suitably amplified, actuate the loud-speaker, which is a device to convert these electric waves into the same sound waves that reached the microphone.

Basic Sight-Sound Differences

In both radio telegraphy and telephony, intelligence is transmitted by modulating the fiow of electric current of high frequency in an antenna.

In considering television communication, it is apparent that the form of intelligence to be transmitted is of a vital nature. Therefore, a way must be found of controlling, or modulating, the highfrequency or radio-frequency current generated to the antenna in some way which can represent the picture to be transmitted. This is where we encounter the basic difference between sight and sound transmission.

In sound broadcasting, we have to transmit only one thing, one bit of intelligence, one sound at a time, with others in sequence. Music or speech consists of merely one sound at a time,



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1945 Theatre Catalog, 4th Edition, Page 592