630 



Popular Science Monthly 



tions of this article apply solely to the 

 proper manipulation of the inductances and 

 condensers. Before beginning to tune, the 

 operator must have made sure that his 

 connections are properly fastened, that his 

 aerial and ground leads are correctly 

 arranged, that the telephones are in good 



The usual inductively coupled type of a receiv- 

 er employed in the majority of wireless sets 



condition and that the detector is adjusted 

 to a sensitive point. The two last-named 

 items are all-important. Each time the 

 receiver is operated they should be tried 

 out by a "test buzzer" equipment such as 

 described in the December, 1916, article of 

 this series. Once these preliminary 'steps 

 are gone through, the entire attention may 

 be devoted to tuning. Unless you are 

 certain that the rest of the apparatus is in 

 working order, however, much time and 

 effort will be wasted in trying to build up 

 signals by tuning alone. 



Tuning the Coupled Receiver 



The tuner of Fig. i has five tuning 

 adjustments. The positions of switches A, 

 B and C govern the wavelength to which 

 the primary or antenna-to-ground circuit 

 is tuned. The third adjustment (switch C) 

 also determines the coupling between pri- 

 mary and secondary. The fourth switch 

 D, and the secondary variable condenser, 

 fix the wavelength to which the secondary 

 is tuned. The more turns of loading coil 

 included in circuit by the switches A and B, 

 and the more turns of transformer coil cut 

 in by switch C, the greater the tuned wave- 

 length of the primary circuit. Also, the 

 more turns cut in by switch C, the closer 

 the coupling between primary and secon- 

 dary and, consequently, the broader the 

 tuning of the set. The more turns of the 

 transformer coil cut into the secondary 

 circuit by the switch D, and the greater 

 the active capacity of the secondary vari- 



able condenser, the longer the tuned wave- 

 length of the secondary circuit. 



Having noted the effects of the various 

 switches and the condenser, as above, and 

 bearing in mind that the object of tuning 

 is to get the primary tuned wavelength and 

 the secondary tuned wavelength to be as 

 nearly as possible equal to the wavelength 

 being received, it is not hard to see how the 

 various elements must be adjusted. There 

 is an additional object, however, which 

 complicates matters a little; that is, the 

 coupling between primary and secondary 

 must be made as loose as possible without 

 sacrificing strength of signals. Since reduc- 

 ing the number of turns cut in by switch C 

 loosens the coupling, it is clear that this 

 switch must be kept as near to zero as 

 possible without weakening the signals too 

 greatly. 



First Operations in Tuning 



It is almost impossible to adjust the five 

 variables to their best points simultaneous- 

 ly, so the best plan is to eliminate certain of 

 them from the preliminary operations. In 

 beginning to tune in a signal, therefore, 

 open the switch which thus disconnects the 

 secondary condenser. This gives the secon- 

 dary a broadly tuned character, and makes 

 it much easier to "pick up" a strange. 

 Next, cut in nearly all 

 inductance by setting 

 switch D to 3. high 

 value of turns. Since 

 the secondary con- 

 denser is cut out, this 



SEC. 



LOADING COIL 



\ 



mcommg message, 

 of the secondary 



FIG. 5 



Loading coils added to the primary and 

 secondary circuits for tuning long waves 



has the effect of bringing the secondary 

 circuit more nearly in tune with the usual 

 wavelengths than would be the case if fewer 

 turns were used. Third, set the coupling 

 switch at forty or fifty turns (unless the 

 wavelength you expect to receive is very 

 short, when fewer coupling turns will be 



