Man's physical universe

xanabras

530 MAGNETISM AND ELECTRIC ITV

readers of this book have made their own crystal-detector radio receiving

sets.

In his earlier experiments, Marconi succeeded in transmitting messages

over a distance of two miles. In 1898 he sent messages from

Poole to Alum Bay, Isle of Wight, a distance of eighteen miles.

1910 Marconi had succeeded in perfecting his apparatus to the point

where messages could be transmitted six thousand miles.

In 1900 Marconi produced a much improved wireless apparatus.

In the first place he added tuning coils, thus removing the interference

caused by the increasing number of stations. The tuning coil is a device

that controls the wave length of the waves sent out so that the

waves of only those lengths desired are sent out.

Today each transmitting station is required by law to use certain

wave lengths in order to avoid the confusion of competing messages

on the same wave length.

Vacuum Tubes Are Now Used to Receive High-frequency Oscillations.

The crystal detectors were replaced by two-electrode vacuum tubes.

Later, three-electrode vacuum tubes were introduced for receiving and

ultimately for sending radio waves.

Thomas Edison made the first discovery that led to the modern

vacuum tube. In connection with studies to determine the reason

why black deposits formed in his

FlG. 260. Apparatus by which the

Exiison effect may be produced.

By

incandescent lamps, he placed

an insulated plate between the

filament and the glass. When

this plate was connected with

the positive terminal of a battery

through a projection

fused

through the glass and the filament

was connected with the

negative pole of the battery, a

current of electricity was found

to flow through the wire. This

effect, known as the Edison effect,

remained unexplained for several years, although Edison rightly concluded

that the current somehow leaped across the gap between the

plate and the filament. Today we believe that this effect was due to

the fact that hot metals emit electrons.

The efficiency of this one-way electron valve was greatly increased

by using a cylinder bent around the filaments instead of the plate.

The three-electrode vacuum tube represented the next great de-

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