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lamp which will cause it to light up. Condensers of variable capacity are widely used in the circuits of wireless apparatus as aids in tuning, and they permit passage of electrical energy in the manner explained above.

493. The receiving station. Fig. 469 represents a "regenera'tive" receiving circuit capable of receiving long or short waves. When the modulated waves (Fig. 462) reach the tuned aërial of the receiving station, they develop therein feeble electrical oscillations which induce oscillations in L of the tuned grid circuit. This varies the potential of the


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FIG. 469. A regenerative receiving circuit

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grid G, thus causing corresponding changes in the strength of the electronic current flowing from the incandescent filament F to the plate P, and thence back through the plate coil PC. The plate circuit is so tuned with respect to the grid circuit that these current variations in the plate coil react inductively on the coil L, connected with the grid circuit to strengthen the original grid-circuit current. This intensifies the variations in potential at the grid, which in turn intensifies the variations in strength of the electronic current from filament to plate, and this still further intensifies the variations in potential at the grid, and so on, up to

the limit of the electron supply in the tube. This is the Armstrong regenerative principle by which very feeble oscillations produced by the incoming waves may be amplified and then used to intensify the original oscillations. The energy for regeneration comes from the battery B. When the tube is in use the grid tends to accumulate a negative charge which, as we have seen (§ 491), would tend to block completely the action of the tube. Therefore, a high-resistance grid leak r is shunted around the condenser C, to permit the return


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FIG. 470. A two-variometer tuned-plate-circuit for receiving short waves



of such a detrimental accumulation of electrons to the filament F, by way of r and L. The telephone receivers used in wireless work contain thousands of turns of very fine wire wound upon iron and because of the consequent chokecoil" effect, or impedance, of these coils for high-frequency changes in current strength, the radio-frequency variations (Fig. 463) of the plate current pass largely by way of the variable condenser C,, while the slower audio-frequency variations (Fig. 464) of the plate current pass readily through the receivers to actuate the diaphragm.

Fig. 470 shows a two-variometer circuit for the reception of short waves. A variometer is a variable inductance used

for tuning and it consists of two coils in series, one of which revolves within the other. If current is passed through the variometer when the inner coil is turned so that its magnetic field combines with that of the other coil to make the greatest resultant magnetic field, the inductance of the variometer is found to have its greatest value and the adjustment is then for the longer waves, or slower oscillations. If the inner coil is now turned through 180°, the resultant magnetic field is at minimum strength; and, because of the small inductance, the variometer is adjusted to the shorter waves. Intermediate positions of the inner coil are used for wave lengths lying between these limits. Complete tuning is accomplished by use of the two variometers, the two variable condensers and the sliding contact on the aërial coil.

494. The transmitting station. The vacuum tube may be used not only as a rectifier, a detector, a modulator, and an amplifier, but under certain conditions as a generator of oscillations varying over an extremely wide range of frequency from less than 1 oscillation per second to 300,000,000 or more per second. Nearly all present-day "broadcasting" is done by use of vacuum-tube generators. For high-power long-distance transmission banks of vacuum-tube amplifiers may be used to throw into an aërial an aggregate power of many hundreds of kilowatts. Indeed, at the present time rapid progress is being made in the experimental construction of power tubes each one of which is capable of giving an amazing output. The life of a vacuum tube is generally from 1000 to 5000 hours, whereas a high-frequency alternator, such as the Alexanderson, will last for many years.

It is entirely beyond the scope of this book to explain the actual details of a wireless-telephone transmitting station. However, the method used at present in high-power longdistance transmission is indicated in Fig. 471 and may be outlined as follows: Air vibrations produced by the voice

make variations in the current of the primary circuit of the telephone transmitter (§ 376). This induces corresponding E. M.F.'s in the secondary circuit, which impresses audiofrequency variations of potential upon the grid of a vacuumtube modulator. The resulting changes of audio frequency in the current of the plate circuit of the modulator correspondingly affect the output of the high-frequency oscillation generator. This modulated radio-frequency output is

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Vacuum-tube modulator

FIG. 471. High-power long-distance wireless-telephone transmitting station amplified by a bank of three-electrode power tubes and is then delivered to the aërial through an oscillation transformer. In broadcasting stations (see opposite p. 429) a weaker and somewhat simpler arrangement of tubes is used.

NOTE. The following reference books will prove helpful to teachers and to those pupils who desire a more complete understanding of "wireless": (1) BUCHER, Practical Wireless Telegraphy, Wireless Press, 326 Broadway, New York City; (2) GOLDSMITH, Radio Telephony, Wireless Press, 326 Broadway, New York City; (3) HAUSMANN and others, Radio Phone Receiving, Van Nostrand Co., 8 Warren St., New York City; (4) MORECROFT, Principles of Radio Communication, John Wiley and Sons, 432 Fourth Ave., New York City; (5) SCOTT-TAGGART, Thermionic Tubes in Radio Telegraphy and Telephony, Wireless Press, 326 Broadway, New York City; (6) Elementary Principles of Radio Telegraphy and Telephony (Radio Communication Pamphlet 1), 79 pages, illustrated, 10 cents, Superintendent of Documents, Government Printing Office, Washington, D. C., 1922.

Although transoceanic telephonic communication has been successfully and repeatedly accomplished (see opposite p. 441), no regular service for such communication has yet been established.

495. The electromagnetic theory of light. The study of electromagnetic radiations, like those discussed in the preceding paragraphs, has shown not only that they have the speed of light but that they are reflected, refracted, and polarized, — in fact, that they possess all the properties of light waves, the only apparent difference being in their greater wave length. Hence modern physics regards light as an electromagnetic phenomenon; that is light waves are thought to be generated by the oscillations of the electrically charged parts of the atoms. It was as long ago as 1864 that Clerk-Maxwell, (see opposite p. 102), of Cambridge, England, one of the world's most brilliant physicists and mathematicians, showed that it ought to be possible to create ether waves by means of electrical disturbances. But the experimental confirmation of his theory did not come until the time of Hertz's experiments (1888). Maxwell and Hertz together, therefore, share the honor of establishing the modern electromagnetic theory of light.



496. The electric spark in partial vacua. Let a and b (Fig. 472) be the terminals of an induction coil or static machine; e and ƒ, electrodes sealed into a glass tube 60 or 80 centimeters long; g, a rubber tube leading to an air pump by which the tube may be exhausted. Let the coil be started before the exhaustion is begun. A spark will pass between a and b, since ab is a very much shorter path than ef. Then let the tube be rapidly exhausted. When the pressure has been reduced to a few centimeters of mercury, the discharge

FIG. 472. Discharge in partial vacua

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