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- 



^^MOI^ 



Fig, 30 



— ™UlMr- 



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, 



319 



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- 



i 



T 



\^ 



^ 



Fig. 31: 

 with a 



A simple wireless sender 

 loaded antenna circuit 



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- 



