6 90 



Professor J. A. Flemmg 



[May 24, 



must have the power of re-starting itself if entirely extinguished for 

 a short time, but this should not take place until the P.D. between 

 the terminals exceeds a certain value, that is, it must not take place 

 too easih or at too low a voltage. If the arc is formed between solid 



;t: 45 



YiQ. 10. — Characteristic Curves for Continuous-current Arc 

 IN Air and Hydrogen (Upson). Arc length = 1-25 mm. 



carbon terminals then it appears that these conditions are only fulfilled 

 up to a certain frequency, that is when employing a rather large 

 capacity in the condenser circuit. We then obtain Mr. Duddell's 

 musical or singing arc, which emits a sound because the rapid variations 

 of current through the arc, by varying the energy expended in it, 

 expand and contract the column of incandescent vapour forming the 

 true arc, and therefore the layers of air next to the arc, and hence 

 send out air waves which are heard as sound. Frequencies up to 

 10,000 or so are possible, although many physicists, such as Banti, 

 Corbino, and also Maisel, contend that much higher frequencies can 



my laboratory, under my direction, by Mr. W. L. Upson. It has been found 

 that for an arc between a cold metal and a carbon terminal in hydrogen for 

 the same length of arc, the rate of decrease of terminal voltage with increase 

 of current is always greater than for an arc in air between two carbon 

 terminals ; in other words, the volt-ampere characteristic is steeper. Also it 

 has been found that in the case of a carbon arc in air the current can be in- 

 terrupted for a much longer time vi^ithout permanently extinguishing the arc 

 than is the case for the metal-carbon arc in air or hydrogen. 



