Page:Popular Science Monthly Volume 88.djvu/485

 Popular Science Montlily

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��ters, sent simultaneously on the same wavelength. Each sender used a diiier- ent spark fre- quency, and three banks of relays, such as those de- scribed, were con- nected in the tele- phone circuit of the receiver. Each o f the relay- groups was me- chanically tuned to the tone-fre- quency of one of the senders, and therefore re- from that station

���Fig. 2. An audion bulb of high voltage

��sponded to signals only.

The use of group-frequency tuning, in addition to the ordinary wavelength tun- ing, gives a vast number of combinations for the reduction and prevention of in- terference. The practical difficulty is that "static" is amplified along with the messages, and, what is most unfortunate, produces a ringing, musical sound. This of course makes it all the harder to read the signals.

Patent No. 1,129,942, 1915, issued to H. D. Arnold, shows a form of audion tube of increased efficiency. It is found possible, by varying the location of the plate with respect to the grid and fila- ment, and by altering the form of the grid, to build audion amplifiers in which the magnified en- ergy is character- ized either by high voltage or high current. A bulb of the high voltage type is shown in Fig. 2, in which the grid

3 consists of fine wire and is placed close to the fila- ment 5. The plate

4 is set at some distance from the grid-filament system and the whole is enclosed in the usual evacuated bulb 6. The patent referred to deals especially with various coml)i- nations of these high and low voltage amplifiers for line telephony; neverthe- less, the use of similar instruments for

���Fig. 3. A quenched spark gap of unusual construc- tion

��both radio transmitters, amplifiers and receivers makes the design of interest.

A quenched spark-gap of unusual con- struction appears in Fig. 3. Small tungsten buttons, having parallel faces, are set into brass or copper electrode- holders, and set with their parallel faces very close together. A number of these gaps, each operating in open-air, are connected in series to make up the com- plete quenched-gap system. With gaps of this type, on account of the very high melting point of tungsten, the two elec- trodes can be adjusted very close to- gether without any great likelihood of

���Fig. 4. Diagram of the shpping-contact de- tector for radio telegraphy

short-circuiting through oxidation. Also, since tungsten is practically unburnable, the diameter of the electrodes may be made very much less than in the ordi- nary quenched gaps. The inventor states that little difficulty is experienced in get- ting pure spark-tones when the tung- sten electrodes are used, because of their constancy in operation ; it is pointed out that even with incorrect coupling values, the spark tone remains good. Oscillation circuit couplings of as high as 45 per cent, giving extremely high quenching, may be used. The drawing is taken from 1915 U. S. patent No. 1,152,272, is- sued to H. Boas.

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