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A Method for Measuring the Pressure of Light by Means of a Thin Metal Sheet
(G. D. West. On a method of measuring the pressure of light by means of thin metal foil. Part I. Proc. Phys. Soc. of London. Vol. XXV, 324, 1913. Part II. Proc. Phys. Soc. of London. Vol. XXVIII, 251, 1916).
The author shows that the pressure of radiation emitted by an electric incandescent lamp with a carbon filament, at a distance of several centimeters from the lamp, is sufficient to produce a microscopically measurable deflection of a very thin metal sheet (aluminium or gold) suspended on a horizontal axis \(BA\) in a special vessel \(H\) (see figure). The author makes measurements of the pressure of light at various degrees of rarefaction of the air inside the vessel, beginning with atmospheric pressure and ending with the highest vacuum attainable by means of coconut charcoal and liquid air contained in the vessel shown at the left.
The results of one of the experiments are as follows.
Experiment with an aluminium sheet in the highest vacuum: length of the sheet 7.43 cm; mass of \(1\) sq. cm of it equals \(1.16 \cdot 10^{-4}\) gram; energy of the lamp at a distance of 10.5 cm: \(1.95 \cdot 10^{-3}\) ergs/sec.
| Distance of the sheet from the lamp | Deflection of the sheet, observed | Deflection of the sheet, calculated |
|---|---|---|
| 10.5 cm | \(2.5 \cdot 10^{-3}\) cm | \(2.6 \cdot 10^{-3}\) cm |
| 11.5 “ | \(2 \cdot 10^{-3}\) “ | \(2.3 \cdot 10^{-3}\) “ |
| 12.5 “ | \(1.5 \cdot 10^{-3}\) “ | \(2 \cdot 10^{-3}\) “ |
At low rarefactions still better agreement is obtained. Thermo-convection currents of the gases have their maximum effect, which interferes with measurements, at pressures of about \(0.002\) cm of mercury; and even at this point their effect is insignificant, since the temperature difference on the two sides of the thin metal sheet, by means of which the pressure of light is measured, is very small. In practice it is more convenient to use rarefactions smaller than this critical pressure of \(0.002\) cm. The nature of the gas in the vessel with the sheet has no special influence on the phenomenon, especially at low rarefactions. On the other hand, the position of the sheet relative to the walls proves to be very important, and only by placing the sheet symmetrically with respect to all the walls can one avoid a special deflection toward the nearest wall, independently of the direction of the incident light.
N. Shchofro.