2G8 PROCEEDINGS OF THE AMERICAN ACADEMY. 



resulting oscillations, and the use of low potentials. The trains of 

 oscillations either follow each other very closely or, as with the arc 

 method, the oscillations are continuous with no intervals of inactivity. 



For the purpose of this article it will be sufficient simply to men- 

 tion and to classify the short gap and arc methods according to the 

 characteristics of the resulting oscillations, and to designate the 

 types by the names of their observers or discoverers. There are 

 three such types, namely : the Duddell or singing arc oscillations ; 

 the Poulsen oscillations ; and the Wien oscillations, or the so-called 

 "Stosserregung." 



The Duddell * oscillations are produced when a direct-current 

 carbon arc in air is shunted by an oscillatory circuit. This method 

 of producing oscillations is characterized by the fact that the arc 

 always remains lighted, that is, the condenser current is at every 

 instant less than the supply or main current. These oscillations are 

 of small energy and limited in frequency to about 10,000 per second 

 which fact renders them impracticable for radio-telegraphy. 



The Poulsen ^ oscillations are produced by the Poulsen arc, which 

 is essentially a Duddell arc between a water-cooled copper anode and 

 a carbon cathode ; this arc takes place in a magnetic field and in an 

 atmosphere of some hydro-carbon gas. In this case the arc is ex- 

 tinguished at each oscillation, the condenser current at some instants 

 being greater than the supply current. The Poulsen oscillations are 

 much more intense than the Duddell type, and much higher frequencies 

 can be attained ; the energy, however, grows rapidly less as the fre- 

 quency is increased. Frequencies of a million or more can be obtained 

 but with small energy. Although the Poulsen oscillations are used in 

 wireless signaling they have some disadvantages, such as low efficiency, 

 and the characteristic change of wave length with changing arc length 

 and arc current. 



Under the head of Wien ^ oscillations fall those produced by the 

 Lepel * arc, the Peukert ^ arc, etc. The generators for oscillations of 

 this class consist of two or more very short gaps between large metallic 

 surfaces separated by paper, or by a thin film of oil, and so mounted as 

 to exclude the air. The gaps are connected in series and shunted by an 

 oscillatory circuit similar to the arrangement for the other forms of 



1 W. Duddell, Joum. Inst. Elect. Eng., 30, 232 (1900). 



* V. Poulsen, British Patent Specifications, 15,599 of 1903. 

 « Max Wien, Physik. Zeitsch., 11, 76, Feb. 1, 1910. 



* Lepel, Electrician (London), 63, pp. 142, 157, 174, 345, 376 (1909); 64, 

 pp. 153, 386 (1909-10); G. W. Nasmyth, Phys. Rev., 32, No. 1, 103 (1911). 



' Peukert arc. Electrician (London), 64, 361, 550. 



