Page:Popular Science Monthly Volume 90.djvu/172

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156

��Popular Science Monthly

��It often happens with double-slide tuners of this type that there are several positions for both sliders which give good signals for a single wavelength. Therefore it is a good plan to try several settings of the antenna-slider, varying the other contact at the same time; thus one can sometimes find a single pair of settings which give

���ANTENNA- SUDER

��6L0CKING-C0NDEN5ER Fie.5

��An arrangement such as this makes it pos- sible to "tune-in" many stations effectively

markedly improved signals. This double- setting effect is more noticeable with other connections than that shown in Fig. 5, but with some combinations of antenna and detector it may be found in this arrangement also. The best thing to do is to have a scale, marked in number of turns or in centimeters of coil, fastened close to each slider, and then to make a tabulation of the best settings for each station as it is heard. Such a table makes it possible to leave the apparatus tuned quite close for any desired outside station, and to feel con- fident that its messages will be received whenever it starts to send.

The Apparatus

For receiving long distances it is merely necessary to combine the crystal-detector and blocking-condenser, recently described, with a pair of good head-telephones, a fairly long antenna, and a double-slide tuner, in order to receive at night from commercial stations hundreds of miles away. It is wise economy to buy good telephones; for, as a general rule, the more money invested in them (so long as they are purchased from a reliable dealer) the greater will be their effectiveness. For receiving from amateur stations, which are required by law to operate on waves less than 200 meters in length, it is not desirable to have an antenna longer than 100 feet or so, though longer wires may be used if a condenser is connected in series, as will be explained in later articles. To get the best

��results from the longer wave stations, such as the commercial plants which use 600 meters and the Naval stations on waves of 1000 and 1200 meters, it is best to have antennas about 200 feet long. Using the ordinary crystal-detector, a double-slide tuner, good telephones and a single aerial wire swung between chimneys 150 feet apart and 40 feet above the ground, it is not unusual to receive messages 600 or 800 miles at night during the winter.

The loading-coil described for the one- mile sender, in the October article, may be used in the diagram of Fig. 5, if two clips are utilized. This will not give a very long range of wavelengths, but will do for experiments. For the best receiving, a modification of the double-slide tuner, which is easily made, will be described next month.

Valdemar Poulsen's Mercury Drop Impulse-Amplifier

THE Danish scientist, Valdemar Poul- sen, has brought into the service of wireless, as well as of cable signaling, a new impulse-amplifier which depends for its action upon the globulizing effect of mer- cury. The explanation of this mercury-

���FIG.1 The amplifier depends upon the effect of a current on the cohesion between molecules

globulizing effect is the same as that of the^ globules of water issuing from an ink-] dropper. It is the attraction of the mole- cules for each other that tends to balancej the tendency of the water within the dropj to break through the surface. Naturally]

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