Man's physical universe

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532 MAGNETISM AND ELECTRICITY

series of vacuum tubes, each of which ampUfies the current that it

receives until there is a sufficient amount to operate a loud-speaker.

A

Fig. 262. Diagram of a simple radio receiving set. A, antenna ; induction

or tuning coils ; C, detector tube ; D, the A battery ; £, the B battery ; F, receiver

G, variable condenser ; H, grid leak ; /, ground ; /, fixed condenser.

A high-frequency radio wave is known as a carrier wave. Carrier

waves may be sent over power lines or telephone wires, and several

messages can be transmitted at one time by using carrier waves of

different frequencies.

Vacuum Tubes May Also Be Used to Generate High-frequency Electromagnetic

Waves.

In the discussion of the use of the three-electrode vacuum tubes as

rectifiers, it was pointed out that such tubes may also be used to produce

alternating currents from direct current.

Herein lies the value of

the three-electrode tube in the transmission of radio waves. If the

current from the plate is fed back to the grid, small oscillations in the

circuit produced by the tuned circuit will be amplified again and again

until an oscillating current of large intensity is produced.

Used as oscillators, such vacuum tubes give oscillations which are

very powerful. The ordinary broadcasting oscillator may generate

from 35,000 to many millions of waves per second; the frequencies

used in the United States range from 550,000 to 1,500,000 per second.

Sound waves in the microphone vary the current in the oscillator

and thus modulate the amplitude of the waves sent out.

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