80 volt 

 arcs. 



the Occurrence of Enhanced Lines in the Arc. 865 



must be an indication that a certain critical value of the 

 electric force has been reached below which these lines are 

 no longer excited by this means. In the following table 

 are given the values of the applied electric fields at the 

 moment of the abrupt falling off or sudden stoppage of 

 the spark line X 4481. These values have been computed 

 from the data given in §§ 4 and 5. It should be specially 

 remarked that they do not represent the drop of potential 

 along the path of the arc flash at the particular moment 

 considered, but only the intensity of the electric force which 

 would prevail if the resistance of the arc were infinitely 

 great. The arc flashes were passed between poles of mag- 

 nesium metal, except where otherwise stated. 



Intensity of 

 Nature of source. applied Eemarks. 



electric field. 



j Glycerin 840 ^^ | Carbon bands strongly 



Paraffin oil 770 „" J developed. 



I Water 528 ,. 



J Concentrated solution of I 



^ magnesium sulphate V 478 ,, 



between Mg poles J 



Same between graphite \ ?-.q Carbon bands not 



poles J " seen. 



Hydrogen 519 ,, 



"^ Blown arc 404 



f Water 462 „ 



200 volt J Concentrated solution of "I Carbon bands absent 



arcs. 1 magnesium chloride be- > 834 ,, but calcium lines 



tween graphite poles ...J predominant. 



As will be seen, the majority of the figures obtained for 

 X4481 are in the neighbourhood of 500 — , and it may 

 be useful to examine more closely those cases which differ 

 widely from this average value — namely, the glycerin arc, 

 the paraffin oil arc, and the arc with a drop of magnesium 

 chloride solution between graphite poles. Our photographs 

 of the spectra of the first two cases reveal the fact, by reason 

 of the very strong development of the carbon bands, that the 

 electric current was largely carried by carbon vapour, which 

 is a good conductor. Hence the drop of potential along the 

 arc flash will of necessity be low on account of the low 

 resistance of the vapour, and the point at which the critical 

 value of the electric force for X 44£1 is reached will be 

 situate nearer to the stationary pole than in an arc of 

 greater resistance. Since our calculations take no account 

 of the prevailing resistance they will, in an arc of high con- 

 ductivity, lead to values which are too great. In the case of 



Phil Mag. Ser. 6. Vol. 43. No. 257. May 1922. 3 K 



