206 DE. W. GEOFFREY DUFFIELD ON THE 



HUMPHEEYS* at pressures of 42 and 69 atmospheres, and measurements were made 

 of the displacement of several lines at those pressures. 



The following is a more detailed discussion of the behaviour of the copper arc 

 under pressure than has yet been published. The work has been confined to the 

 region \ = 4000 to X = 4600 A.U., and the pressures have ranged from 1 to 203 

 atmospheres, t 



2. THE APPAKATUS. 



The arc was formed between copper poles, diameter -| inch, within the pressure 

 cylinder designed by Dr. PETAVEL, F.R.S., which had previously been used for the 

 investigation of the effect of pressure upon the iron arc.J The light passed through 

 the window in the side of the steel chamber, and was reflected by the mirror system 

 (previously described), which enabled the image of the arc, which was very unsteady 

 at high pressures, to be continually focussed upon the slit of the 21^-ft. Rowland 

 Grating Spectroscope in the Physical Laboratory of the Manchester University. 



The Second Order Spectrum was employed, the dispersion being 1'3 A.U. per 

 1 mm. 



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

 a gas-holder, suitable valves and gauges being interposed. 



Direct current was supplied by the Corporation mains at 100 volts, the pressure 

 being reduced to about 60 volts by a resistance frame ; when the copper arc was 

 burning steadily under pressure the current was maintained at about 13 amps. 



3. THE BEHAVIOUK OF THE COPPER ARC UNDER HIGH PRESSURES. 



Under normal pressure the arc between copper rods was maintained with 

 comparative ease, and up to 5 atmospheres pressure no difficulty was experienced 

 with it, but at higher pressures it rarely burned for more than three or four seconds 

 at a time, because the luminous metallic vapour which was expelled from it rendered 

 it very unstable and frequently blew it out ; at these pressures a single exposure is 

 the integrated effect of a number of short-lived arcs. 



Like the iron arc, the brilliance of the copper arc increases with the pressure of the 

 surrounding air, the image of the arc upon the jaws of the slit becoming dazzlingly 

 bright at 100 atmospheres. As far as could be gauged from visual observations the 

 intensity increases continuously with the pressure. 



In spite of this increased brilliance there is no concomitant decrease in the 



* HUMPHREYS, ' Astrophysical Journal,' XXVI., p. 18 (1907). 



t The description of the work upon pressures above 100 atmospheres is given at the end of this paper 

 (p. 222). It was added October 23, 1908. G. D. 



t W. G. DUFFIELD, 'Report Brit. Association York,' p. 481 (1906); 'Koy. Soc. Proc.,' A, 79, p. 597 

 (1907); 'Phil. Trans.,' 208, p. Ill (1908). 



