Man's physical universe

xanabras

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

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