From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Experiments with gas-filled lamps, discussing lifespan, pressure, and gas disappearance.
| Identifier | ExFiles\Box 79\1\ scan0260 | |
| Date | 19th February 1930 | |
| - 4 - 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 electrodes while running, which are re-absorbed 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 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 differs 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. The disappearance of gases of the helium group in spectrum ( Contd.) | ||