Page:Popular Science Monthly Volume 90.djvu/648

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��Popular Science Monthly

��of settings (primary, coupling and secon- dary) are physically interlinked and that whenever you change any of them you must try the single-turn primary switch and the secondary condenser in order to find out whether you are hearing the loudest possible signals.

In picking up very weak signals it is sometimes necessary to adjust the large primary and secondary steps together, since in that way the circuits are kept more nearly in tune all the time, and weaker signals can be heard. Under such condi- tions, it is evident that when few primary turns are in circuit the secondary switch must be near its zero point, and that as the number of primary turns is increased the number of secondary turns must be in- creased correspondingly. Such detailed handling of the apparatus can only be

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���A circuit with switches to connect variable condenser in series or shunt with primary

learned by experience, however, and for cases of this kind these instructions can do little more than suggest a line of action.

Receiving Long Waves

For tuning to very long waves it is some- times necessary to add loading coils to primary and secondary circuits, as shown in Fig. 3. The addition of active turns to the primary of course increases its wave- length range; a similar effect is had in the secondary. The wavelength of the secon- dary circuit may be increased by enlarging the size of the secondary variable condenser, and when this is done the secondary loading coil is of course unnecessary. Neverthe- less, it is a good plan to keep the inductance of the secondary large, and its condenser correspondingly small; the additional load- ing coil is therefore the preferred method of extending the tuning range to include the very longest waves. When the main transformer is very small and the received waves are exceeding long, it may not be

��possible to get close enough coupling unless the primary and secondary loading coils are placed near together so that they act as an additional transformer. When this is done, care must be taken that the direction of connection is correct; otherwise the induc- tive effect between the loading coils may neutralize instead of helping that between the original primary and secondary coils.

Another way of getting long wavelengths with a comparatively small receiving trans- former is shown in Fig. 4. Here a second variable condenser is shunted across the primary coil terminals, which has somewhat the effect of increasing the capacity of the antenna. This arrangement has a number of advantages, among which is that by its use the fine tuning of the primary can be accomplished by varying the condenser, and that, as a result, it is unnecessary to build the primary coil with a single-turn switch. Further, and especially when very small receiving antennas are used, signals may be louder with this shunt primary condenser than when the same long wave is received by adding the primary loading coil. In Fig. 4 the secondary loading coil is shown, but, as before, its effect of increasing the tuned wavelength may be obtained by enlarging the secondary condenser.

Tuning to Short Waves

When it is desired to receive wavelengths which are short compared with the funda- mental wavelength of the antenna, it is very convenient to insert a variable condenser in series with the aerial connection, as shown in Fig. 5. This has the effect of reducing the size of the receiving antenna, and makes tuning to short waves a very simple matter. As when the primary variable condenser was used in shunt (Fig. 4), small primary inductance steps are not needed, for the sharp tuning may be secured by means of the series condenser. With the arrange- ment of Fig. 5 it is also possible to tune to wavelengths of the medium range, since the inductance in series may be increased to give the period desired. When used in this way the signals are sometimes weaker than those obtained from the arrangement of Fig. 2, but the tuning is usually sharper, because the ratio of inductance to capacity is increased.

The primary circuit is shown in Fig. 6, which may be arranged with two single- pole double-throw knife switches so as to connect the variable condenser in series or in shunt with the primary, or to cut it out

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