WIEELESS TELEPHONY— PESSENDEN. 175 



In 1905 Professor Fleming " invented a very efficient detector based 

 on the " Edison effect " in incandescent lamps, and the observations 

 of Elster and Geitel ^ on the rectifying effect of such an arrangement 

 on Hertzian oscillations. 



Virtually nothing was done in Europe in the way of producing sus- 

 tained oscillations by the arc or high frequency method until re- 

 cently, possibly because of Duddell's erroneous statement '' to the 

 effect that frequencies much above 10,000 could not be obtained by 

 the Elihu Thomson arc method, and Fleming's statement" that an 

 abrupt impulse was necessary and that high frequency currents, even 

 if of sufficient frequency, could not produce radiation. 



In 1903 Poulsen « invented an interesting modification of the Elihu 

 Thomson arc, which consists in forming the arc in hydrogen instead 

 of in air or compressed gas as previously done. This modification is 

 not, however, so efficient as the older methods and gives oscillations 

 varying in amplitude and intensity and accompanied by strong har- 

 monics,^ but I have considered it worth mentioning on account of the 

 amount of interest it appears to have excited in Europe. 



Some very important and interesting papers on electrical oscilla- 

 tions were published during these years by Oberbeck,^ Wien,'' Drude,* 

 and Bjerknes.-' 



In America the development of the sustained oscillation nonmicro- 

 plionic system has proceeded steadily and it may now be said to have 

 reached the stage of commercial practicability. On account of the 

 amount of work which has been done it is impossible to refer to more 

 than a few^ of the recent advances. 



The following are some of the later types of detectors : 



The frictional receiver,^" in W'hich the waves produce a change of 

 friction between two moving surfaces and so cause an indication. 



The heterodyne receiver,' in which a local field of force actuated 

 by a continuous source of high-frequency oscillations interacts with a 

 field produced by the received oscillations and creates beats of an 

 audible frequency. 



" Fleming, Proceedings Royal Society London, 1905, vol. 74. 



* Elster and Geitel, Wied. Ann. de Physik, vol. 52, p. 433. 

 ''Duddell, The Electrician, 1903, vol. LI, p. 902. 



^ Fleming, Proceedings of the International Congress, St. Louis, 1904, vol. 3, 

 1.. 003. 



e Poulsen, United States patent No. 789449, June 19, 1903. 

 f Austin, Bulletin of the Bureau of Standards, vol. 3, No. 2. 

 ffOberbeck, Wied. Ann. der Physik, vol. 55, 1895. 

 ''Wied. Ann. der Physik, vol. 8, 1902. 



* Drude, Ann. der Physik, vol. 13, 1904. 



} Bjerknes, Ann. der Physik, vol. 44, 1891, and vol. 47, 1892. 

 *^ United States application No. 251538, March 22, 1905. 

 ' United States application No. 271539, June 28, 1905. 



