304 Mr Aston, Neon Lamfs for Strohoscopic Work 



The disappearance of gases of the HeHum group in spectrum 

 tubes is invariably associated with sputtering of the electrodes 

 which, at high pressures, only takes place when the gas is spectro- 

 scopically free from chemically active gases. It is generally sup- 

 posed that the gas so disappearing remains embedded or adsorbed 

 in the layer of sputtered aluminium on the sides of the tube near 

 the cathode, the idea of true chemical combination not being 

 acceptable without very rigorous proof. 



In order to obtain information on this point, a completely run 

 out specimen of the first batch of lamps, which was of course very 

 heavily sputtered, was taken for test. First the sputtered cathode 

 end was gradually heated to near the softening point of the glass 

 (when it cracked) without any substantial or apparent increase in 

 the internal pressure of Neon. The end was then cut ofi, broken into 

 small pieces and heated in a quartz tube in a high vacuum apparatus 

 provided with a spectrum tube. At a temperature about the 

 softening point of the glass a good deal of gas was released which 

 showed the hydrocarbon spectrum (but may nevertheless have 

 contained some Neon as this is easily masked) ; this gas was pumped 

 off and on heating further to a red heat, as the glass started to 

 melt, Neon was given off, the spectrum showing quite clearly. 



Apparatus for measurement and analysis of the gas so released 

 was not available, but it is hoped to repeat this interesting experi- 

 ment, which shows definitely that the Neon is contained either in 

 the sputtered aluminium or very near the surface of the glass so 

 that it is released by heat. 



Use of other Gases instead of Neon. 



Ordinary chemically active gases give very feeble illumination, 

 CO being about the best. Helium gives a bright discharge but not 

 nearly so valuable in quality for visual work as Neon ; its presence 

 as an impurity in the latter gas renders the discharge more rosy 

 red but up to 10 per cent, does not affect its brightness seriously. 

 Mercury vapour as used by C. T. R. Wilson in his photography of 

 ionisation tracks would probably give very bright flashes, but the 

 fact that the lamp has to be kept very hot is a serious objection. 



Reason for Superiority of Neon. 



The brilliant orange-red glow of the discharge in Neon is com- 

 posed almost entirely of lines in the region 5700-6700 a.u. and is in 

 such striking contrast to sunlight that strohoscopic observations 

 can even be done in broad daylight if necessary, the ordinary 

 appearance of the rotating disc having merely a grey background 

 added, looking bluish by contrast. 



The actual amount of light radiated per unit of energy, i.e. 



