Popular Science Monthly 



621 



of each, and because the simple capacity 

 of the tuning condenser is not the only 

 capacity in the circuit. For instance, in 

 Fig. 2 it is necessary that the secondary 

 effective capacity shall be 0.00173 mfd.; 

 this is not the value of C] itself, since 

 the capacity added by the presence of 

 the detector, stopping - condenser and 

 telephones must be considered. The 

 capacity of the detector is very small 

 and, since the stopping-condenser and 

 telephones are in series with the de- 

 tector, the resultant added capacity is 

 still smaller. If, instead of the arrange- 

 ment shown, the telephones had been 

 connected across the detector, the limit- 

 ing capacity would have been that of 

 the 'phone windings, which is sometimes 

 fairly large. A good reason for placing 

 the telephones in shunt-to the blocking- 

 condenser instead of in shunt to the 

 detector is therefore brought out; the 

 detector capacity is so small that tuning 

 is governed almost entirely by the tun- 

 ing-condenser Ci when the arrangement 

 of Fig. 2 is used. 



A two-circuit tuner is shown in Fig. 

 3. It has all the elements as in Fig. 2, 

 with the addition of potentiometer P 

 and battery B for 

 adjusting the de- 

 tector R to its point 

 of maximum recti- 

 fication efficiency. 

 The tuning to in- 

 coming waves is ac- 

 complished as in 

 Fig. 2 ; the antenna 

 circuit is first tuned 

 by adjusting the in- 

 ductances until its 



resonant frequency agrees with that of 

 the waves desired, and then the sec- 

 ondary circuit is tuned to the same fre- 

 quency by proper adjustment of in- 

 ductance L3 and capacity Ci. It 

 should be noted that the same arrange- 

 ment of telephones is shown here as in 

 Fig. 2; the potentiometer, battery and 

 telephones arc connected across the 

 stopping-condenser C, and not directly 

 across the detector R, so that their 

 capacity will not become prominent in 

 the tuning of the secondary. This ar- 

 rangement, as compared to the more 

 common connection, gives greater ease 

 of adjustment over a wide range of 



T 



wavelengths, and makes sharper tuning 

 possible. 



The same principles of tuning may be 

 applied to direct-coupled apparatus, as 

 shown in Fig. 4. Here the primary and 

 secondary are made part of the same 

 coil, the proper amounts of inductance 

 for each being tapped off by moving the 

 sliding or switch-contacts as shown at 

 L2 and L3. Obviously, the sum of Li 

 and L2 gives the amount of primary or 

 antenna-circuit inductance, and the in- 

 ductance of L3 is that used in the sec- 

 ondary. To tune the secondary circuit 

 to the desired frequency L3 and C\ 

 must be used; Li and Ln tune the 

 primary. The coupling between pri- 

 mary and secondary is determined by 

 the distribution of the total antenna- 

 circuit inductance between the coils Li 

 and L2. For any given wavelength, the 

 larger Li becomes, the smaller is L^ 

 (since it is necessary that their sum shall 

 remain the same) and the looser the 

 coupling between primary and sec- 

 ondary. The less of coil L, is used, the 

 more of L2 it becomes necessary to cut 

 into circuit, and the closer the coupling. 

 With a direct-coupled apparatus of this 

 sort, having a sep- 

 arate primary load- 

 ing-coil L,, it is 

 possible to secure 

 as exact tuning as 

 with the inductive- 

 ly-coupled appara- 

 tus; the bad rcfnifa- 

 tion of "two-slide" 

 tuners, as to dull- 

 ness of (lining, has 

 arisen mainly be- 

 cause the coupling is so tight that only 

 broad tuning can be had when all the 

 primary inductance is directly part of 

 the coil which also forms the secondary. 

 In many cases it is not necessary to 

 have as sharp selectivity as may be 

 secured from the circuit of Fig. 2; in 

 these instances the secondary (iim'ng 

 condenser C, may be dispensed with, as 

 shown by Fig. 5. Here the primary L, 

 and the loading-coil L, are adjusted as 

 usual to the wavelength which it is 

 desired to receive; the secondary is so 

 broadly tuned, however, that it is not 

 necessary to regulate its inductance by 

 small amounts in order to secure loud 



Fig. 4. The primary and second- 

 ary are parts of the same coil 



-4- 



fLp 



