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
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��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
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