Page:Popular Science Monthly Volume 88.djvu/650

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��Popular Science Monthly

���signals. If a single-pole switch is placed in the lead to Ci of Fig. 2 (as shown in Fig. I of p. 306 in the February issue) it becomes possible to use either the broad or the sharp-tuned secondary system, as may be desired. For any given wave- length more induc- tance on the sec- ondary will be re- quired to get loud signals with the ar- rangement of Fig.

5, than for Fig. 2; "^ this is because the

secondary circuit of Fig. 5 actually is broadly tuned by the capacity of the detector, blocking-condenser, telephones, etc., acting with the total inductance of the secondary. Since the natural capac- ity of these other elements is small, a larger secondary inductance is made necessary to reach the desired wave- length.

Where still less closeness of tuning is necessary, the arrangement of Fig. 4 may be modified by omitting the loading- coil Li which permits variation of coupling, and by doing away with the tuning-condenser Ci, as shown in Fig. 6. This results in the ordinary close- coupled direct tuner, which is useful for picking up signals when interference is not severe. By connecting in the tuning- condenser Ci, as shown by the dotted lines, it is possible to improve the selectivity of the system in some meas- ure, especially if the blocking-condenser C2 is made of very small capacity or even left out altogether.

It will, of course, be seen at once that in tuning the secondary circuit of any of the receivers described above, one may choose a great many combinations of inductance and capacity in order to have resonance to a certain frequency. For instance, the wavelength of 5000 meters is reached when the secondary inductance is 4 millihenrys and the condenser 0.00173 mfds. If the inductance were 2 millihenrys, twice the former capacity, or 0.00346 mfd. capacity would be re- quired. The best ratio of inductance to capacity depends largely upon the type of detector used; for most crystals, the

��Fig. 5. By opening the

secondary tuning - condenser

the primary may be tuned

independently

��condenser may be about 0.003 mfd.,

maximum for wavelengths from loco

to 5000 meters, and correspondingly

smaller or larger

for shorter or longer

waves. For the au-

dion, where the

highest possible

voltage should be

applied to the grid,

it is best to use

comparatively large

values of secondary

inductance, with

the corresponding

small secondary

condensers; Ci, for

had best never be

mfd., even for the

��1

��c'

��audion working, larger than o.ooi longest waves.

The size of the stopping-condenser C2 is also a matter of interest. For crystal detectors, it is customary to use capaci- ties of from o.oi to 0.04 mfd. at this point in the circuit. By making the stopping-condenser variable in steps of about 0.005 mfd., it is possible to select a best value for each particular operating condition; in general, the higher the telephone resistance and the higher the incoming spark-frequency, the smaller the stopping-condenser may be. The smaller this condenser is made, after it passes below about o.oi mfd., the less is the damping of the secondary circuit, and the sharper is the tuning. Too great reduction of the capacity, however, in the attempt to gain selectivity, re- sults in weakening the response to the signals. The size of the blocking or grid-circuit condenser for the audion is much less than for the crystal detectors; Co is then best made variable, with a range including values as small as 0.000 1 mfd. or less.

In operating any of the sharply tuned circuits shown in the foregoing, it must be remembered that the best settings of primary inductance, coupling, and sec- ondary inductance and capacity are largely dependent upon each other. In tuning-out interference and "bringing in" a particular station, the best plan is first to open the secondary tuning- condenser circuit to give the arrange- ment of Fig, 5 ; this makes it possible to tune the primary independently and ac-

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