Page:Popular Science Monthly Volume 88.djvu/642

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��Popular Science Monthly

��location and skill, from the Navy station at Darien, which is about 1800 miles from Washington. He may also receive signals from spark stations many hun- dreds of miles farther away than before, and "bring in" the stations which he has been hearing from two to ten times as loudly as before.

The accompany- ing illustrations portray an arrange- ment of three tun- ed circuits, the open (antenna) cir- cuit, the secondary circuit, and the au- dion wing (high po- tential) circuit. The various condensers and inductances shown are the usual tuning devices.

The antenna in use with the set here described consists of two stranded copper wires 250' long, spread 4' apart on bamboo spreaders, raised 30' above level ground, located up in the mountains of northeastern Pennsylvania. The anten- na, with the aerial tuning inductance, primar}- of inductively-coupled tuner and the condenser shunted around the aerial tuning inductance and the primary winding, permits of tuning to resonance with from 1000 to 8000 meters wave- length. Tuning to shorter wavelengths may be done by cutting in the condenser shown in the ground wire. With this set it has been easily possible to read signals from the Navy station at Darien, Panama, day or night, ever since the station was opened.

The constants of the aerial, inductan- ces and condensers may be varied, of course, and those in use or on hand in •most advanced amateur stations may, perhaps, serve the purpose; but in the aggregate a fine degree of resonance must be procured. All inductances (save those of the tuner) are in oak boxes; the tubes are fastened to the lids by brackets, with the switches on top of the lids. This arrangement permits the entire unit to be lifted out of its box without disturbing connections, if desired. Tap leads inside are covered with soft rubber tubing. Coil boxes may be placed on end to save space.

The tuner is of the familiar inductively- coupled navy type; the inductances in

���How the inductance coils are made

��both tuner circuits are variable by switches only, 16 ten-turn and 18 one- turn in the primary, and 12, equally spaced, in the secondary. Inductance i is made of a paper mailing tube, 3" outside diameter, 18" long, and wound closely over 16" of its length with No. 25 DCC wire. Including the ends of the wind- ing, ten taps, equal- ly spaced, are led to a ten-point switch. Inductance 6, in the secondary circuit, is made of a paper mailing tube, 3" by 18", wound closely for 16" with No. 36 DCC wire; and 10 equally spaced taps are led to a ten-point switch. Inductance 13 is iden- tical with inductance 6. Inductance 14 is of the same dimensions but it is wound with No. 25 wire for use when tuning to shorter wavelengths, and to permit fine variations in conjunction with inductance 13 when tuning to long waves. All con- densers (except that around the high- potential battery) are of the familiar segmental variable type, with range of capacity from 0.0008 to o.ooi mfd. Condenser 3 is filled with castor oil, giving it a maximum capacity of approxi- mately 0.004 mfd. It is used to boost the wavelength of the antenna circuit.

Rheostats 9 and 10 are employed to regulate the filament voltage. One is the ordinary rheostat that is a part of every audion detector. It has a total resistance of about 10 ohms. The other has a total resistance of only i}/2 ohms in 10" of length. This second rheostat is not absolutely essential, and may be omitted, but it has been found to be very convenient to have such a rheostat for closely regulating the lighting voltage, whether storage or dry cells are used. The condenser 15 shunted across the high potential battery is of the ordinary tele- phone type, of from i to 2 mfds. capacit^^ Condenser 18 may be either a true variable, or a variable-fixed condenser susceptible of several changes of capacity. The function of these condensers is to provide paths of low impedance, for the high-frequency currents, around the high- potential battery and the telephones. It is considered good practice to have a

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