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Page:Popular Science Monthly Volume 90.djvu/647

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Popular Science Monthly


��used) so as to make the coupling quite close and therefore to aid in hearing signals to which the apparatus is not accurately tuned. Fourth, set the single-turn switch B at the middle of its arc, and finally try to pick up your desired signals by moving the ten-turn step primary switch A. As



��Another way of procuring long wavelengths with a receiving transformer of medium size

soon as signals are heard, reduce the num- ber of coupling turns by switch C, at the same time compensating for those cut out of the primary circuit by correspondingly increasing the turns of the loading coil. When the best settings of switches A, B and C are found (that is, the p)oints upon which the signals are loudest), close the secondary condenser switch and set the secondary contact D to the button giving the loudest signal. Then adjust the con- denser itself until the signals are loudest; and the receiver will be about at its best adjustment.

Further improvement in signal intensity can be secured by readjusting the primary single-turn switch, B, after the secondary is sharply tuned. Sometimes it is best to change the position of the coupling-switch C after the secondary condenser is cut in. All this must be determined by trial for each particular station heard. The impor- tant things to remember are, to make the rough primary adjustments first, the secon- dary adjustments second, the final coupling adjustments third, and to tr>' both the primary single-turn switch B and the secondary condenser last, in order to make 1 further improvement in signal strength or I tuning sharpness, every time any other i adjustment is changed.

The Inductively Coupled Receiver

j ^ The circuit of Fig. 2 is the usual induc- I tively coupled type used in most receiving

��sets. It is entirely satisfactor>' for general receiving, but must be handled carefully in order to give the best results. Its manipu- lation corresponds very closely with that just set forth in detail for the auto-trans- former tuner. The only radical difference is that coupling is changed by moving the primary coil physically with respect to the secondary. As before, the more turns of primary cut in between the single-turn switch B and the ten-turn switch A, the greater the tuned primar>^ wavelength. Similarly, the more turns of secondary coil cut in by switch D, and the greater the active capacity of the variable condenser, the greater the secondar>- wavelength. Again, the farther apart the primar^-^ and secondary coils, the weaker (looser) the coupling between them and, consequently, the sharper the tuning and the less inter- ference difficulties.

The objects of tuning are, as before, to adjust the primary' and secondary tuned wavelengths to agree with that being received, while at the same time keeping the coupling as loose as possible without destroying the signals. The plan to follow, therefore, is to op)en the secondary condens- er switch and cut in a large portion of the secondary' coil as before. Then close the coupling by sliding the coils well together. Finally, search for the desired signals by making rough adjustments of the primary.

VWhen signals are heard, open the coupling by sliding the ""Sr S? ^^^^ farther apart and at the y / same time adjust both the

���A variable condenser inserted in series widi the aerial connection for short wavelengths

primary switches to the point where signals are loudest. Then reduce the number of secondary turns and cut in the secondary condenser, setting these to the points which give best signals. For the last adjustment, open the coupling still further and tune the primary and secondary' still more accurately. Always bear in mind that the three groups

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