206 DK. w. GEOFFREY DUFFIELD ON THE 



1. Preliminary. The apparatus and method of taking photographs of the spectra 

 of metallic arcs under pressure have been described in previous papers.* The arc 

 was formed between two poles of the metal (not quite pure) |-inch diameter and 

 about 6 inches long, which were enclosed in a steel cylinder, the design of Prof. PETAVEL, 

 F.R.S., capable of resisting a high internal pressure. The light from the arc passed 

 through a window in the side of the cylinder, and was reflected by a system of mirrors 

 upon the slit of the 21 g-foot Rowland grating spectrograph in the Physical Laboratory 

 of the University of Manchester. 



As in previous experiments, the spectrum of the second order was used, the 

 dispersion being 1'3 Angstrom Unit per 1 mm. 



An increase in pressure was obtained by the admission of air into the cylinder 

 from a gasholder, suitable valves and gauges being interposed. 



The arc was fed by continuous current from the Corporation mains, which supplied 

 100 volts ; this was reduced to about 50 volts at the terminals of the arc. 



2. Behaviour of the Nickel Arc under .Pressure. As in the case of the gold arc 

 the ease with which the arc burned depended both upon the coolness of the poles and 

 the freshness of the air supply. The arc was maintained for short intervals without 

 difficulty: 



Of electrodes previously used the nickel arc behaved more like that between copper 

 poles. The intensity increased very markedly with the pressure, but photometric 

 measurements were again out of the question, owing to the unsteadiness of the arc. 



At low pressures the nickel arc was distinctly mauve in tint, but it became whiter 

 as the pressure of the surrounding air was increased. 



3. The PJiotographs : (l) Region Investigated. -The investigation of the spectrum 

 extended from A = 3450 to A = 5500 : as this range of wave-lengths extends over 

 about 162 cms., it was necessary to move the 50-cm. camera into four different positions. 

 This involved a large amount of labour, as it practically quadrupled the amount of 

 work which would have been necessary had it been possible to photograph the whole 

 of the spectrum at once. 



The photographs were taken partly during October, 1908, and the remainder during 

 April, May, and June of 1910, the series being interrupted by the writer's absence in 

 Australia. 



The following table shows the pressures at which photographs have been taken in 

 different regions of the spectrum : 



* W. G. DUFFIELP, 'Phil. Trans.,' A, vol. 208, p. Ill, 1908 (Iron Arc); vol. 209, p. 206, 1908 

 (Copper Arc); vol. 211, p. 33, 1910 (Silver Arc); vol. 211, p. 51, 1910 (Gold Arc). 



