302 Mr Aston, Neon Lamps for Strohoscojnc Work 



tion indicated in the figure than in the opposite, hence it effectually 

 stops the "reverse" current from the secondary of the coil. 



Other important results depending on the length of the fila- 

 ment will be discussed later, it should be roughly one hundred 

 times the length of the spark the coil is capable of giving in air 

 when running on the tuning-fork break. 



It is hardly necessary to state that the shape into which the 

 filament is wound is not in the least essential and could be varied 

 to any extent in lamps for special purposes. 



The electrodes are of aluminium and may be of any form so 

 long as they are not too small. 



Method of Filling Lamps. 



As Neon, like the other gases of the Helium group, has the 

 remarkable property of liberating gas from aluminium electrodes 

 which have been completely run in for other gases, the operation 

 of filling necessitates the contamination of a comparatively large 

 volume of Neon, so that this can only be done economically and 

 conveniently where liquid air is available for re-purifying. 



So far all the lamps have been filled on the author's Neon 

 fractionation apparatus at the Cavendish Laboratory^, The gas 

 for filling is contained in charcoal cooled in liquid air. A quantity 

 is admitted to the exhausted lamp which is then sparked at a 

 pressure of 1 to 3 mm. with a small coil for a time. The dirty gas 

 is then pumped off with a Toepler mercury pump, a fresh supply 

 of pure gas admitted and the tube run again. These operations 

 are repeated until spectroscopic and other observations show the 

 desired conditions of purity have been reached and are not altered 

 seriously by prolonged running. The full charge of 5 to 10 mm. of 

 gas is now let in and the lamp sealed off. The whole operation takes 

 about 3 hours, three lamps being filled at once. The pressure, 

 purity and time of running in are all matters of some nicety as 

 will be seen from consideration of the life of the lamp. 



Life of the Lamps. 



Apart from accident the lamps are serviceable until the pressure 

 of gas within them becomes too low for the spark to light them 

 adequately. Their life appears to consist of two distinct periods, 

 the first during which chemically active impurities derived from 

 the electrodes and walls of the tube are being slowly and completely 

 eliminated (at least as far as a spectroscopic observation goes) and 

 the second during which sputtering of the cathode takes place and 

 the inactive Neon itself slowly disappears until the pressure gets 

 too low for use. During the first period the luminosity steadily 



1 V. Lindemann and Aston, Phil. Mag. sxxvii, May 1919, p. 527. 



