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).
Analysis of the nature and duration of a neon lamp's 'working flash' for stroboscopic use.
| Identifier | ExFiles\Box 79\1\ scan0262 | |
| Date | 19th February 1920 | |
| D.R. 238A (100 T) (S.G. 642. 19-2-20) G.{Mr Griffiths - Chief Accountant / Mr Gnapp} 2818 - 6 - Bancels have shown ("Photochemie de la{L. A. Archer} Retine," Jl. Phys. Path XIII, 1911) that the Fovea Centralis of the eye is immensely more sensitive to red light than the outlying portions of the retina, thus a Neon lamp as a source of general illumination is very disappointing, but when viewed directly appears surprisingly bright. As the spinning disc of the stroboscope subtends a comparatively small angle the Fovea is the only part of the observer's eye used in testing, which is probably the reason for the eye strain with the spark. NATURE AND DURATION OF THE "WORKING FLASH." If one analyses the flash of a short spectrum type Neon tube in a rotating mirror it is seen to consist of two separate parts, an extremely short flash followed by a flame or "arc". The first is probably due to the simultaneous ionisation of the gas throughout the whole length of the tube, the second to the further carriage of current by the ions formed during the first. The structure of the latter, which appears to consist of bright striations travelling from anode to cathode at velocities of the order of that of sound in the gas, is of great theoretical interest and is at present under investigation. Discussion of its nature is needless in the present paper for its duration being of the order of thousandths of a second it is useless for stroboscopic work, and, by the employment of a sufficiently long filament tube, it can be eliminated altogether. In a lamp properly proportioned to the power of the coil in use the whole energy of the discharge is absorbed in the first flash. In order to get some idea of the duration of this "working flash" the following experiment was performed. A plain mirror silvered outside to avoid double images, was mounted vertically on the axis of a large centrifuge and the image in this of the Neon lamp at a distance of 3 metres was observed by means of a telescope with a micrometer eye-piece. Each division in the micrometer subtended 4.2 x 10⁻⁴ radians and when the centrifuge was running at 3500 revolutions per minute corresponded to 5.75 x 10⁻⁷ seconds. The lamps were run with the tuning-fork attachment used in actual testing and were viewed directly and also through ground glass with a V-shaped slit to be certain of getting the effect x The difference of retinal effect between red and green light can be easily observed by looking at an ordinary luminous wrist watch in the faint red light of a photographic dark room. On shaking the watch so sluggish is the green light in recording its position on the retina compared with the red that the figures seem to be shaken completely off the dial, giving a most curious and striking effect. ( Contd. ) | ||