Page:Popular Science Monthly Volume 91.djvu/467

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Popular Science Monthly

Sustained Wave Telegraphy Between as shown by AB. the United States and Germany

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��BEFORE 19 13 there was no direct means of communication between Germany and the United States, except by submarine cables. The Allies cut these cables im- mediately war was declared. Between that time and our own break with Germany, communication was established by two of the most important pairs of wireless sta- tions ever known. And now that all active amateur and

���Curve representing progressive values of the cuirent generated by a radio alternator

semi-professional work has been discon- tinued, this ought to be just the time for our friends to study the "workings" of these great stations.

The quenched singing spark systems, which were used for almost all radio work at the time these stations were erected, were first tried at Sayville, but proved entirely too feeble for the distance that had now to be covered, about 4000 miles. The sustained-wave system was therefore installed at Sayville, and it was found to be reasonably successful. Exceedingly high frequency sustained alternating cur- rents were used in all four stations. For generating these, the stations at Eilvese (Germany) and Tuckerton (New Jersey) used the Goldschmidt "reflection" system, while those at Nauen (Germany) and Sayville (Long Island) used the Arco-Joly "multiplication" system.

The secret of the success of sustained- wave telegraphy lies in the fact that high frequency alternating currents — that is, the currents which go through many changes in direction in a second — are better radi- ators of energy from the sending sta- tion, while they also enable the receiving instruments to respond more strongly. As an alternating current generator re- volves, the strength of the electromotive force it produces varies as shown in Fig. 1. The strength of the electromotive force first increases until it reaches its maximum

��It then decreases and finally becomes negative, reaching a maxi- mum negative value CD. It then reduces to zero again and becomes positive in value once more. By greatly increasing the speed of the generator or by increasing its number of poles it is possible to increase greatly the number of times the electro- motive changes in direction per second, and in this way, to increase greatly the radiation efficiency of a station. However, due to the fact that a machine cannot easily be made sufficiently strong in the one case and that the electrical efficiency may be too greatly reduced in the other case, this simple machine method was not used for these high power stations. The transformer methods of Goldschmidt or of Joly were therefore adopted, rather than the radio frequency dynamo of Fessenden and Alexanderson, used so extensively be- fore the war.

The Goldschmidt system depends on the phenomenon called "reflection." That is, the currents set up in the rotating part of the alternator (the rotor) can be "reflected" back into the stationary part of an alterna- tor (the stator) so that the frequency of the original rotor current is increased to double the original. This can be made to take place by a suitable electrical connec- tion on the stator. Again, the induced current set up in this manner in the stator can be "reflected" back into the rotor. The frequency of this new current last set up will be increased to three times the original frequency. Finally, on reflecting

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Goldschmidt "reflection" alternator for quad- rupling the frequency of the original current

this once more into the stator, the resulting current frequency will be once more in- creased, and to four times the fundamental. This is the current that was used in the antenna.

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