Page:Popular Science Monthly Volume 92.djvu/335

 Popular Science Montldy

��the tone slightly below the next C above middle C, and is much used in radio telegra- phy. Both of these are audio frequencies. Frequencies of above 10,000 cycles per second are called radio frequencies, for the reason that they are most useful in produc- ing radio waves. Wireless telegraph trans-

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��Fig, 30

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A closed circuit with alternator, and a closed oscillation circuit for alternating currents

mitters use frequencies as high as several million per second, which are, of course, far above the upper limit of sound or audio frequencies. Since currents of these enor- mously high frequencies are used in the antenna circuits of wireless transmitters, the problem under consideration becomes how to generate such electrical move- ments.

Machine Generation of Alternating Current

There are two practical methods of pro- ducing alternating currents over large ranges of frequency, as indicated in Figs. 29 and 30. The first of these shows an alternator or alternating current generator G connected in series with a coil of wire or inductance L and a resistance represented by R. The generator G usually consists of a machine in which coils and magnets are moved relatively to each other at comparatively high speeds, so that the coils have induced in them voltages which change in intensity and di- rection from instant to instant. The series of alternating voltages thus produced, when applied to the circuit, first in the direction of the arrow X and then in that of Y, gives rise to an alternating current through the resistance R and inductance L. The fre- quency of this alternating current depends entirely upon the frequency of the voltage impulses; if the voltage is applied 500 times per second in the direction of either arrow, the current will have a frequency of 500 cycles per second. For any given strength of voltage, the amount of current will depend upon the amount of effective inductance and resistance in the circuit, and will be less,

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the larger the inductance and resistance. The number of times the voltage impulses in one direction are applied per second, or the frequency, depends upon the construc- tion of the generator G; the higher its speed, or the greater number of magnetic poles and corresponding coils it has, the higher the frequency of the current. This mechanical method of direct generation is used almost exclusively for production of the commer- cial alternating currents at frequencies from 15 to 500 per second. For radio transmit- ters, special generators which produce fre- quencies as high as 200,000 per second are built and used. Still higher frequencies can be reached by machine generation of this type, particularly if ^ome sort of fre- quency transformation is involved.

Generation by Condenser Discharge

The second important method of gen- erating alternating currents is that of the condenser discharge, as shown in Fig. 30 Here an electrical condenser C is connected in series with the resistance R and induc- tance L, to some extent taking the place of the generator G in Fig. 29. If we imagine an electric charge to be placed upon the condenser plates and the circuit then to be closed as in Fig. 30, it is not hard to realize that the voltage impressed on the circuit by the condenser charge will cause a current to flow in one direction, say that of the arrow X. The interesting feature of the arrange- ment though, is that when the resistance is not too large as compared with the capacity and inductance, the current will keep on flowing after the condenser has discharged itself fully, dnd will in fact recharge the condenser to some extent in the opposite di- rection. By proportioning the cir-

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��cuit properly, the recharging may be made to reach a value almost as high as the initial potential of discharge. Manifestly, when the second maximum is reached the condenser will once more discharge through the induct-

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