510 



3fr. W. DudcMl 



May 17, 



potential difference : the arc is no longer an unstable conductor, and 

 oscillations cannot be produced. 



There are various ways of rendering the arc unstable, even when 

 the current varies rapidly. One of these consists in using very small 

 currents. For instance, metal arcs using currents under one-half of 

 an ampere are extremely unstable, and remain so even under high 

 frequencies (see Figs. 12 and 14, which give some curves I have 

 obtained for arcs between carl)on and copper electrodes respectively). 



Another method is to energetically cool the electrodes ; for 

 instance, to make one of the electrodes consist of a vessel containing 

 water, or squirt a jet of water through the arc. The third method, 

 due to Mr. Poulsen, consists in placing the arc in hydrogen, or some 

 hydrogen compound, such as coal-gas or alcohol vapour. By this 

 means high frequency oscillations can easily be produced up to aliout 

 a million per second, and at the lower frequencies of, say. 100,000, 

 considerable powers can be dealt with. 



'-^^OTj i /Q^9^ 



Fig. 15. — Poulsen Abc Connections. 



To render the arc still more unstable, Mr. Poulsen places it in a 

 strong transverse magnetic field (Fig. 15). With an apparatus of 

 this sort I am informed that some 80 k.w. or more can lie dealt 

 with by a single arc, and if several arcs are placed in series large 

 powers can be transformed. Fig. 16 is a photograph of a 12 k.w. 

 arc generator. 



If the apparatus is properly adjusted the liigh frequency currents 

 produced are very nearly true alternating currents, such as are ol)tained 

 from an alternator. 



The only difficulty I find in working the apparatus is to maintain 

 the frequency absolutely constant, as it appears that the frequency 

 depends to a small extent on the current through the arc and on the 

 arc length. In the actual apparatus employed for wireless telegraphy 

 the electrodes are kept in rotation so as to ensure a greater constancy 

 in the arc length. 



It must be remembered that if the arc is burnt in coal-gas instead 

 of the electrodes burning away, carbon is actually deposited, and the 

 carbon electrode increases in length. 



