Man's physical universe

xanabras

540 MAGNETISM AND ELECTRICITY

The energy passing through a phototube frequently releases a

amount of energy from a battery or dynamo by means of a

larger

relay, which is a kind of electromagnetic switch. It may also be

amplified and stepped up by vacuum tubes.

Lights can be turned on and ofT by the use of phototubes when day

changes to night. Trafific lights can be operated by intruders crossing

an invisible beam of ultraviolet or infrared light. Garage doors can

be opened by shining the lights of an automobile on them.

Phototubes are also widely used to measure the intensity of illumination,

as mentioned in a previous Section.

Sound Pictures Use the Phototube.

Sound motion pictures were made at one time by synchronizing

phonograph records with the motion pictures, but the advent of the

photoelectric cell made possible the present very much more satisfactory

type of sound picture. When the moving picture is made,

sound vibrations open and close a shutter which allows a narrow beam

of light to shine on the emulsion. The sound vibrations are thus

recorded on a narrow strip at the side of the film, called the "sound

track." In the projection machine there is a pencil of light which

shines on this sound track and through it on a phototube. The phototube

thus converts variations of light intensity into variations in the

intensity of an electric current. This current is then amplified and

sent through a loud-speaker by the use of equipment similar to that in

a public-address system.

Television Is an Application of the Photoelectric Effect.

It is obvious that the fluctuation in current from a phototube can

be transmitted by a wireless broadcasting station and then be picked

up and amplified by a receiving set just as easily as sound waves are

broadcast and received. If the amplified current in the receiving set

is sent through an electric light sensitive to current variation, instead

of through a loud-speaker, this light w'ill show variation in intensity

corresponding to the variations in the intensity of the original light

showing on the phototube. Neon tubes show a satisfactory fluctuation

of light with the fluctuation in the current.

The major problem that remains is to send variations in light from

many points rather than from only one point. This is accomplished

in one television system by means of "scanning" disks, which contain

holes arranged in a spiral. When this disk is revolved in front of an

resolved into a large number of light

illuminated object, the latter is

and shadow points. It is necessary for the eye to receive impressions

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