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XXIII. On a Comparison of the Arc and Spark Spectra of 

 Nickel produced under Pressure. By E. G. Bilham *, 

 BSc, A.R.C.Sc. 



IN a recent paper t, Prof. W. G. Duffield has made a 

 detailed comparison between the results of his investiga- 

 tion of the effects of increase of pressure upon the arc 

 spectrum of nickel J, and those obtained by the present 

 writer §, in which the mode of excitation was the spark 

 between nickel electrodes connected to the secondary coil of 

 a resonance transformer. Many interesting points of simi- 

 larity are to be observed, but one's attention is immediately 

 arrested by certain very remarkable differences. 



The most noteworthy feature of the results is that, whereas 

 the mean displacements towards the red of the lines in the 

 spark spectrum are, upon the whole, in reasonable agreement 

 with the means derived from the arc spectrum — assuming 

 direct proportionality of displacement to increase of pressure, 

 — the displacements in the arc under ten atmospheres 

 pressure are, on the average, about twice as great as in the 

 spark spectrum. An exception is to be observed in the case 

 of lines which broaden slightly but symmetrically under 

 pressure, without reversal, i. e., lines of Class III. in Gale 

 and Adams' notation. These lines appear to suffer approxi- 

 mately equal shifts in the two cases. For other unreversed 

 lines the results are somewhat discordant. This is not 

 surprising when one considers that the diffuseness of the 

 lines under pressure renders the measurements rather un- 

 certain even under the most favourable circumstances. 



From the point of view of making a rigorous comparison 

 between the results of the measurements of the displace- 

 ments produced under the two conditions, it was somewhat 

 unfortunate that the pressure used for my main series of 

 experiments (namely, ten atmospheres above the normal 

 pressure) should have happened to coincide with a region 

 where Duffield observed an abnormally high rate of shift 

 per atmosphere. The means at my disposal prohibited the 

 use of pressures much higher than ten atmospheres. Apart 

 from the fact that very radical alterations in the design of 



* Communicated by the Author. 



t Phil. Mag. Sept. 1915, p. 385. 



X Duffield, u Effect of Pressure upon Arc Spectra,'" No. 5, Nickel 

 X 3450 to X 5500, Phil. Trans. A 215, p. 205. 



§ Bilham, " The Spark Spectrum of Nickel under Moderate Pres- 

 sures," Phil. Trans. A 214, p. 359. 



