920 Eesearch Staff o£ the G. E. C, London, on the 



current in the gas-filled lamp, due to increase in thermionic 

 emission from the cathode, will not be accompanied by any 

 increase in current in the vacuous lamp. On the other hand, 

 if both lamps are vacuous, and their filaments approximately 

 similar, the ratio of the filament resistance will hardly change 

 with the current, and the very small space current will not 

 change at all ; the bridge will remain balanced while the 

 current through it is varied over a considerable range. 

 By means of this simple device of the two lamps connected 

 in a bridge through which the current can be altered, it is a 

 matter of a few seconds to determine whether the pressure 

 in a lamp is above or below that at which the glow potential 

 is equal to the applied potential. The glow potential varies 

 so rapidly with the pressure in this region, and the pressure 

 corresponding to the potentials used is so small (about 

 '0002 mm.), that for our purpose this pressure may be always 

 taken as zero, independently of the applied potential. It 

 may be remarked in passing that the disappearance of a 

 visible glow is not always a safe test that the lamp is 

 " cleaned-up," and that the pressure is below the critical 

 value indicated by the bridge. 



In order to measure by this method the quantity of gas that 

 can be absorbed with a given amount of phosphorus, a series 

 of lamps, each containing the same amount of phosphorus, is 

 filled to a series of pressures. Each lamp is then ' ; burnt " 

 on the bridge in series with a perfectly evacuated lamp of 

 the same type, and the division found between those which 

 "clean-up " and those which do not. The pressure P at the 

 point of division can be determined by some ten observations 

 within 10 per cent, of its value ; the mass of gas absorbed 

 is then p V, where p is the density of the gas at pressure P 

 and V the volume of the lamp. 



5. It was established that P, corresponding to a given 

 amount of phosphorus, was independent within wide limits 

 of factor (5), the nature of the discharge producing the 

 absorption. P did not depend on the voltage applied to the 

 lamp, at any rate if this voltage was well above the limit at 

 which appreciable thermionic emission from the filament 

 begins. On the other hand, the time occupied by the 

 absorption increased rapidly as the voltage, the temperature 

 of the filament, and the space current were decreased. At 

 voltages near the limit at which any discharge could be made 

 to pass, it appeared that P decreased slightly, probably 

 because, when the discharge lasts too long, the phosphorus 

 accumulates in the parts of the vessel where no discharge 



