Popular Science Monthly 



631 



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 



SEC. 

 LOADING Cai 



FI6.4 



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 



