Mr Aston, Neon Lamps for Stroboscopic Work 303 



improves, remaining almost constant afterwards till near the end 

 of the second period when it rapidly decreases. 



The first set of lamps were filled with very carefully purified 

 Neon at 1-2 mm. pressure and run till sputtering had commenced 

 before being used; they may therefore be considered to have had 

 no first period at all. These lamps had a life of 500-1000 hours. 



Experiments soon showed that the less preliminary running 

 and the higher the pressure of filling the longer the life would be, 

 but on the other hand, if the preliminary running is not sufficient 

 the impurities derived from the electrodes turn the light' of the 

 lamp a dull grey and render it absolutely useless and pressures 

 above 10 mm. are not advisable as these increase the spark 

 potential of the lamp too much. 



One lamp was actually so nicely balanced in these respects 

 that though it became grey and useless after about 1 hour's use it 

 completely recovered its original brightness after a day's rest. This 

 is clearly a case of carbon compounds being given off by the elec- 

 trodes while running, which are reabsorbed on standing and there 

 is little doubt that were it worth while very prolonged running 

 would render this lamp quite satisfactory. Very slow production 

 of gases from the electrodes is advantageous, as prolonging the 

 first period of the life, so that these should be of a fairly solid 

 pattern. 



So far, the best results have been obtained from a batch of 

 lamps filled at about 10 mm. pressure, some with pure Neon, 

 some with a mixture of Neon and about 10 per cent. Helium. 



One of the latter had a working life of well over 3000 working 

 hours. Helium disappearing from its spectrum after the first few 

 hundred. 



As there is every reason to assume that for any given lamp the 

 life is determined by the total number of coulombs passed through 

 it, the light obtained per coulomb should be arranged to be a 

 maximum. This will be the case when the filament is made as long 

 as possible, consistent with the potential available from the coil. 



Cause of Disappearance of Gas from the Lamps. 

 The exhaustion of gas by continuous running has long been 

 observed in the case of spectrum discharge tubes. It is doubtless 

 allied to the phenomenon of "Hardening" in X-ray bulbs, but 

 difi'ers from the latter in that under the relatively high pressures 

 in spectrum tubes, and the Neon lamps under consideration, the 

 mean free-path of a charged molecule is so small that it can only 

 fall freely through a potential of a few hundred volts and so never 

 attain the very high velocities reached in the X-ray bulbs which 

 are supposed to cause the gas molecules to become permanently 

 embedded in the glass walls. 



VOL. XIX. PART VI. 21 



