Page:Popular Science Monthly Volume 90.djvu/646

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��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

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��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

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