542 MAGNETISM AND KLKCTRK ITY silver particles and are not discharged until a beam of electrons passes over them. This brings about electrical discharges, capable of transmission, in the metal layer under the particles. The original scene is reproduced again by playing a beam of electrons rapidly back and forth across a fluorescent screen, which will shine at the point where an electron hits it. The variations in the stream of electrons are produced by the variations in the current from the receiving set. In 1940 Philo T. Farnsworih patented a new type of television system, in which the electron beam plays on a metal target and then produces X rays which then strike a fluorescent screen and produce an X-ray image. This system makes possible much larger pictures than other systems permit. Television requires such an enormous frequency band that the only space available is in the ultrahigh-frequency region. As the frequencies of radio waves increase, they act more and more like light rays in that they do not bend beyond the horizon as ordinary radio waves do. For this reason the average television broadcasting station will have a range of from thirty to fifty miles, depending on the height of the transmitting antenna and the nature of the terrain. Telephone lines will not transmit television programs, but they can be transmitted by the very expensive coaxial cables. The coaxial cable consists of a tube surrounding a wire, from which it is insulated with hard-rubber spacers. Such a cable can carry 240 telephone messages or 2880 telegraph messages simultaneously. Television in natural color has already been accomplished in the laboratory. The principle of the process depends upon the use of three filters, red, green, and blue, at the transmitter and receiver to pick out, transmit, and receive each color separately and then mix them on the television receiver screen. Light Causes Other Electronic Effects in Certain Substances. Thus, There are other electronic devices which are sensitive to light. photoconductive devices are based on the fact that certain substances, such as selenium, change in electrical resistance when exposed to light. Selenium, for instance, greatly decreases in resistance when light shines on it. For some time selenium cells have been used to turn lights on at sunset and turn them off at sunrise. Selenium cells are unstable, not constant in performance, markedly affected by temperature changes, easily damaged by intense light, and relatively slow to respond to changes of light intensity. Another kind of photoelectric cell is the type which produces a current of electricity without the aid of an outside potential such as
THE PHOTOELECTRIC EFFECT 543 is needed for the types of cells studied thus far. These devices are called "photovoltaic" cells and are very useful for measuring illumination, inasmuch as the current generated is sufficient to be registered by a microammeter, thus providing direct readings. These cells are widely used as "light meters" for ^ photography and other purposes. It is possible to replace several million workers with photoelectric counting and sorting devices. In industry, photoelectric devices are already used to sort out automatically cans which pass through a j labeling machine without labels, Fig. 266. A photo-voltaic cell, and to sort out cigars. Excessive This photo-voltaic cell is used to belching of smoke from a stack is eliminate guess-work in determining f^*" exposures in photogra- detected by photoelectric cells. In t'?^ ^^^ 1 TTii j^7i-i T- I Phy- (Courtesy of the Weston Electhe Holland Vehicular Tunnel, tricallnstrument Corporation.) photoelectric cells are used to count the vehicles, to turn on the fans when needed, and to warn toll-takers to refuse admittance to trucks which are too high to clear the roof of the tunnel. Photoelectric cells are also used to match colors. One application of color-matching is the process control of coffee-roasting in which a light is directed upon the beans as they pass a window in the roaster. The quality of the light reflected from them depends upon how well done they are. When the beans are the right color, they are automatically ejected from the roaster. The above uses of photoelectric cells are only a few of the many now in existence. The new photoelectric phonograph introduced in 1940 replaces the needle of an ordinary phonograph with a sapphire jewel which transmits vibrations to a mirror which reflects light to a photoelectric cell. Friction is reduced in this new phonograph, and therefore records last longer. The sound range is also greatly extended, thus giving more faithful reproduction. The next few years will undoubtedly witness a much more widespread use of photoelectric cells. The social and economic consequences of the photoelectric cell cannot be disregarded. STUDY QUESTIONS 1. Discuss the modern applications of photoelectric cells. 2. Discuss the possible social significance of the widespread application of photoelectric cells. 3. What are the two types of photoelectric cells? 4. What is the photoelectric effect? 5. What is the function of the phototube in sound pictures?