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] 



