Cady and Arnold — Electric Arc. 399 



or 3 mm long extended. At this point the bluish striations 

 began, growing more distinct and nearer together as the anode 

 was approached. 



The stratified discharges mentioned by Gassiot,* de la Rue 

 and Miiller,t and Child, ^ were doubtless manifestations of the 

 same phenomenon. From some of the beautiful photographs 

 published by de la Rue and M uller it is clear that they had to 

 do with an arc discharge in which the anode was not heated 

 to boiling. In fact they express the view that the arc may be 

 regarded as a modified stratified discharge. This, as we have 

 now seen, is strictly true only of the arc on the first stage. 



The small size of the striations would render the experi- 

 mental determination of the distribution of potential between 

 the electrodes very difficult. An approximate value of the 

 potential drop for each striation may be estimated on the 

 assumption that the cathode drop is about 10 volts, the anode 

 drop, judging from the observations of Skinner, § on the order 

 of 30 volts. In a typical case, with 164 volts across the arc, 

 current 0*42 amp., and length 15 mm , with six striations, a simple 

 computation gives about 20 volts as the average potential drop 

 necessary for ionization in each striation. This is less than 

 the drop per striation usually found in the glow discharge, 

 and is probably to be accounted for by the high temperature 

 of the gas. 



§26. Carbon. — We have seen that three characteristic curves 

 may be drawn for the discharge between metals. For carbon 

 terminals still another stage exists, namely the hissing arc, in 

 which the oxidation of the anode causes a further decrease in 

 voltage. 



We have succeeded, by suitably varying the current and 

 the surrounding gas, in obtaining all four types of discharge 

 between carbon electrodes. The experiment was begun in 

 nitrogen with a 460 volt supply. Under certain conditions 

 striations were visible on both the glow and the first stage of 

 the arc. After the current had been increased until the sec- 

 ond stage had arrived, air was slowly substituted for the nitro- 

 gen and the current further increased up to the hissing point. 

 Owing to the heating of the bell-jar, observations had to be 

 made rapidly, so that no systematic series of readings could be 

 taken. The change from first to second stage was not well 

 marked, for the volatilization of the carbon anode seemed to 

 set in gradually as the current increased. Child fl mentions 



*Pogg. Ann., cxix, 133, 1863. fPliil. Trans., clxxi, 65, 1879. 



JPhys. Eev., xx, 364, 1905. §Wied. Ann., lxviii, 752, 1899. 



|Phys. Eev., xx, 373, 1905. 



Am. Jour. Sci.— Fourth Series, Vol. XXIV, No. 143. — November, 1907. 

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