the Occurrence of Enhanced Lines in the Arc. 305 



origin and meaning. From the results of our present ex- 

 periments we may already safely derive the following- 

 fundamental facts : — 



1. The presence of hydrogen is not essential for the ex- 



citation of spark lines. 



2. Spark lines attain a high degree of development only 



in a medium which offers a comparatively high 

 resistance to the flow of electricity. 



3. When hydrogen is present in the medium, either in 



the free state or in combination, its spectrum re- 

 sembles that which is only observed with high 

 tension condenser discharges — namely, its lines are 

 symmetrically broadened as though under the in- 

 fluence of a strong electric field (Stark effect). 



With the help of these facts we will now attempt to 

 derive some notion as to the conditions which underlie the 

 emission of spark lines (4th type enhanced lines) both in 

 spark and arc. As is well known this particular type of 

 lines is most strongly developed in a powerful capacity- 

 spark passing between cold metal electrodes in air at 

 Mtmospheric pressure. IE one or both electrodes are raised 

 to a high temperature the discharge becomes less violent, 

 and the spark lines are considerably reduced in rekitive 

 intensity*. The heat given off by the electrodes raises the 

 conductivity of the air-gap between them, and the condenser 

 discharge takes place at a much lower potential gradient 

 than through cool, and therefore less conducting air. 

 Again, in the self-induction spark the whole of the condenser 

 discharge current passes through air and metal vapour which 

 have been ionized already by the initial discharge (pilot 

 spark). The spark lines in this case are either absent or 

 only feebly developed and confined to the immediate vicinity 

 of the poles. Thus the results with spark discharges teach 

 us that the spark lines are brought out when the discharge 

 passes through a medium vdiich to begin with is only feebly 

 ionized, so that a high potential gradient is required in order 

 to force the electric current through it. As for the arc, it 

 has been shown in this paper that spark lines are not emitted 

 in the carbon arc when the cathode electrode is white-hot. 

 On the other hand, when a graphite pole is used which does 

 not heat to the same extent, spark lines become visible near 



* C. C. Schenck, Astroph}-sical Journal, vol. xiv. p. 130 (1901). 

 Phil. Mag. S. 6. Vol. 43. No. 254. Feb. 1922. X 



