34 DK. W. GEOFFREY DUFFIELD ON THE 



, IWmtmwy-THK effect of pressure upon arc spectra was first investigated by 

 HUMPHRY and MOHLER,* and later by HUMPHREY^ They found that, m general 

 the lines broadened, were diaplaced towards the red end of the spectrum, and showed 

 a greater tendency towards reversal. HUMPHREYS' work has dealt more fully with 

 metals other than silver, of which the following table comprises all previous measure- 

 inents .under pressure : - 



The displacements are in Angstrom units. 



The following investigation was undertaken for the purpose of extending the 

 work to higher pressures. The spectrum of the silver arc has been photographed 

 under pressures ranging from 1 to 201 atmospheres in the region X = 4000 to 

 X = 4600 A.U. 



2. The Apparatus. The arc was formed between silver poles, diameter f 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,} and the Copper Arc. 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^-foot 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 gasholder, suitable valves and gauges being interposed. 



The arc was fed by current from the Corporation mains, which supplied 100 volts 

 continuous, and this was reduced to about 50 at the terminals. 



3. The Behaviour of the Silver Arc under High Pressures. The continuous 

 current arc between silver poles in air at atmospheric pressure was maintained without 

 difficulty until the poles got thoroughly hot, when the convection currents became so 

 violent that they frequently blew it out. The striking of this arc was more easily 

 accomplished than in the case of iron or copper, where the formation of a non-conducting 

 oxide necessitated breaking through this layer before the current could pass. 



* HUMPHREYS, ' Astrophys. Journ.,' VI., p. 169, 1897. 

 t HUMPHREYS, ' Astrophys. Journ.,' XXVI., p. 18, 1907. 

 t W. G. DUFFIELD, 'Phil. Trans.,' A, vol. 208, p. Ill, 1908. 

 W. G. DITJ-FISLD, 'Phil. Trans.,' A, vol. 209, p. 205, 1908. 



