Man's physical universe

xanabras

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

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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?

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