Page:Popular Science Monthly Volume 88.djvu/643

 Popular Science Monthly

��condenser across the telephone terminals with an audion detector, however used.

The leads to condenser 15 must be amply protected against any possibility of shortcircuiting the high-potential battery. If a telephone-type condenser is used, after the leads are soldered to the lugs (which are close together) it is a good plan thoroughly to cover the lugs, the solder, and the wires which are exposed, with sealing wax or paraffin. Condenser II must be one that does not "contact" inside, for a shortcircuit of the high- potential battery is possible when all inductances are tuned out. It is a good idea to insert a 3^ ampere fuse in the high- potential battery circuit.

Condenser 1 1 is the one most handled in tuning. It is used in conjunction with inductances 13 and 14 to bring the wing circuit into resonance with the secondary circuit, and cause the bulb to oscillate. In the wiring diagram, 16 shows a volt- meter across the filament terminals; though not positively necessary, it is useful. If one gets accustomed to regu- lating the lighting voltage with a volt- meter, the likelihood of "crowding" the filament is almost eliminated, and hence, a longer life of the bulb may be expected. The bulb may be shaded to prevent strain on the eyes. In the diagram, 17 is a pole-changing switch provided to reverse the lighting battery ; it is a very desirable adjunct. Any d.p.d.t. battery switch will do.

Condenser 7, usually used to tune the secondary, is not necessary with this set. Tight coupling is employed with long waves. Any necessary variation of ca- pacity for short waves may be effected by slightly changing the coupling. The fixed condenser usually found within each audion detector, in series with the grid, should be removed or bridged over; the variable condenser 8 takes its place in this set.

Assume, now, that we have assembled these component parts and wish to "pick up" Tuckerton, Arlington, or Sayville — stations working with long wavelengths of from 6000 to 8000 meters:

Couple the tuner closely; throw in all of the aerial tuning inductance and those of both primary and secondary of the tuner; throw in all of the secondary loading inductance; set condenser 8 at

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about half capacity (according to the scale); set condenser 11 at zero; throw in all of inductance 13 and about half of 14, adjust the lighting and high-potential

���Arrangement of complete receiver

batteries as usual, then slowly turn up condenser 11.

The lamp should begin to oscillate, and this will make itself manifest by a peculiar muffled "boiling" sound and a change in the sound of static. A loud or troublesome hissing sound indicates too much high-potential or lighting voltage, or both, and should be avoided; the bulb is not in its most responsive condition when this is present. A very slight blue glow is usually observable in the bulb, back of the wing, when it is doing its best. If immediate results and signals are not secured, raise and lower the high- potential and lighting voltages in various combinations and manipulate condenser II until the bulb oscillates. Swinging condenser 8 through its arc, and chang- ing the polarity of the lighting current may have important effects — it depends upon the bulb.

Not all audions oscillate with equal facility, but I have never handled one that would not oscillate with a little patient persuasion. Holding a lighted match to the bulb until the glass is ver%' warm tends to break down its unwilling- ness to oscillate. The sensitiveness of a given bulb while oscillating seems to be directly comparable to its sensitiveness in ordinary use. Since not all audions, nor even both filaments in any one audion, are equally sensitive, this should be kept in mind so that one will not expect an insensitive bulb to give the finest results, under any conditions.

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