632 



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 



/^— -1 JL'MT. 



1 



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 



