312 



Popular Science Monthly 



binding post F. The secondary circuit 

 includes that part of the transformer coil 

 cut in by the "secondary" switch, and runs 

 from binding post H through the detector 

 and stopping-condenser to the ground 

 binding post F. The telephones are 

 connected across the stopping-condenser. 

 A crystal detector and stopping-con- 

 denser suitable for this use were described 

 in the Popular Science Monthly for 

 November, 191 6, in the third article of this 

 series. The proper sort of telephones to 

 use was also discussed in that article. A 

 test-buzzer arrangement for adjustment of 

 the detector was explained in the December 

 article, and should be combined with the 

 outfit of Fig. 7 in order to make the adjust- 

 ment easier and more positive. 



Tuni7ig 



The circuits of Fig. 7, with the apparatus 

 described, will give excellent results in 

 receiving from the commercial and naval 

 spark stations, if used with an 

 antenna of from 150 to 250 ft. 



umm COIL 

 / 



These circuits with the apparatus de- 

 scribed give excellent results in receiving 



length. For shorter wavelengths than 500 

 meters the antenna used should be 

 somewhat shorter, and for waves longer 

 than 2500 meters (that on which time 

 signals are sent) it is advantageous to 

 use still longer wires. It is not neces- 

 sary to erect multiple wire aerials for 

 receiving, nor is great height essential. A 

 200-ft. single wire, of No. 10 gage copper, 

 or even galvanized iron, swung horizontally 

 between two 50-ft. masts or trees, will 

 prove entirely satisfactory for most pur- 

 poses. When it is desired to receive the 

 short waves from amateur stations, which 

 are restricted by law to wavelengths under 

 200 meters, a wire not much over 70 ft. in 

 length should be used. 



In attempting to "pick-up" signals with 

 the outfit of Fig. 7, the best plan is to set 

 the "units" switch on 5, the "coupling" 



switch on about 30^ and then to vary 

 simultaneously the "tens" and the "second- 

 ary" switches. As the turns on the loading 

 coil are increased in number, those of the 

 secondary should also be increased. When 

 signals are heard, the best point of the 

 secondary is selected, and the loading coil 

 and coupling switches adjusted to give the 

 loudest responses. One must of course be 

 careful that his detector is adjusted proper- 

 ly before starting to tune; for this purpose 

 the test-buzzer is a great time-saver. 



It will be noted that more turns of the 

 loading coil and of the secondary are needed 

 for long wavelengths than for short ones, 

 and that when the number of "coupling" 

 turns is reduced, the number of turns in 

 the loading coil must be proportionally 

 increased. This is because the loading coil 

 and the coupling turns in series form a 

 primary circuit, whose effective length must 

 be adjusted for the various wavelengths in 

 the manner described last month. It will 

 also be noted that all stations which have 

 the same wavelength will "come in" best 

 with approximately the same settings, and 

 that the wavelength of any station may be 

 estimated roughly by considering the num- 

 ber of turns in the loading and coupling 

 coils which give the strongest signal from it. 



A thing which is very important in the 

 operation of this tuner, however, is not 

 likely to be evident from the first tests made 

 upon it. That is the relation between the 

 sharpness of tuning and the number of 

 turns on the coupling part of the trans- 

 former coil. Careful observation will 

 bring out that when the number of 

 turns on the coupling section is reduced, 

 and the loading coil correspondingly in- 

 creased to the tuning point, better selectivi- 

 ty is obtained. Usually there is a best 

 value of the coupling turns for every station 

 or wavelength, and its use requires the 

 proper corresponding settings of the loading 

 coil and secondary switches. Often when 

 there is interference it is best to use still 

 fewer turns of the coupling section, corre- 

 spondingly increasing the loading coil, so 

 as to get sharper tuning in spite of a 

 weakened signal. The judicious selection 

 of values for these three coil sections 

 (primary, coupling and secondary), and 

 the proper balancing of signal strength 

 against sharpness of tuning, is one of the 

 items which is most important in commer- 

 cial radio-telegraphy. Many operators fail 

 to get the most out of their receivers merely 

 because they fail to pick out the best 



