W. G. Cady — Electric Arc "between Metallic Electrodes. 239 



Art. XXY. — On the Electric Arc "between Metallic Elec- 

 trodes ; by W. G. Cady. Third Paper." 



IV. The Properties of Glow-Arc Oscillations. 



Ix the second paper it was shown that when a discharge at 

 small current takes place under an impressed e.m.f. of several 

 hundred volts from a metallic cathode, in a gas which is 

 preferably a mixture of hydrogen and acetone vapor, rapid 

 electrical oscillations are generated, of a frequency depending 

 essentially on the electrical constants of the portions of the 

 circuit in the neighborhood of the discharge tube. The nature 

 of the pulsations seems to be a series of rapid changes back 

 and forth between arc and glow. 



In the following paragraphs the properties of these oscilla- 

 tions, as far as they have been examined, will be discussed. 



§15. Apparatus. — Preliminary observations showed it to 

 be desirable to employ a discharge tube with electrodes of as 

 simple a form as possible, in order that the oscillations might 

 not encounter a needless amount of resistance or of self-induc- 

 tance close to the discharge itself. To this end, brass rods 

 A. A (fig. 1), 3 mm in diameter, were extended through corks in 

 the ends of a glass tube about 10x3 cta . The electrodes 

 usually employed were discs about l cm in diameter, with 

 slightly rounded faces. A number of discs of different metals 

 but of the same size and shape were made, which could be 

 screwed on to the rods. 



A mixture of hydrogen and acetone vapor was led in 

 through the glass tube G and out again at 1), where it burned 

 in a minute flame. E was a small side-tube, closed with a 

 cork, for -the insertion of a brush for cleaning the electrodes. 

 As only atmospheric pressure was used, no special sealing was 

 necessary, other than a little soft wax. 



The positive electrode was hardly attacked at all by the 

 passage of the current, but the cathode showed a small cavity 

 at each point from which the discharge had taken place. It 

 was necessary to rotate the cathode slightly from time to time, 

 as well as to adjust the length of discharge (a few tenths of a 

 millimeter) with great care. Hence one of the brass rods was 

 sealed into a glass tube E, which was connected by means of 

 a straight ground-glass joint to a second tube G, the latter 

 being sealed into the cork. By having the two rods slightly 

 eccentric in their mounting, and by rotating E or moving it 

 slightly in or out, very delicate adjustments were possible. 



* Continued from this Journal, vol. xxviii, p. 102, 1909. The paragraphs are 

 numbered consecutively in the second and third papers. 



