Full Text
From Current Literature
On the Expedient Choice of a Length Standard
At the present time, the standard of length is considered to be the distance between two strokes marked on the prototype meter. Comparison of this standard with another line standard, carried out by direct—
Modern sources of monochromatic light
| Type of lamp | Cadmium | Mercury (isotope 198) | Krypton (isotope 84) |
|---|---|---|---|
| Operating temperature (abs.) | 590 | 290 | 65 |
| Relative Doppler broadening (millionths) | 1.65 | 0.85 | 0.6 |
| Maximum order of interference (hundreds of thousands), calculated | 6 | 11.7 | 16.5 |
| Maximum order of interference (hundreds of thousands), measured | 3.9 | 9.2 | 14.6 |
| Path difference (cm) | 25 | 50 | 80 |
| corresponding wavelength (Å) | 6438 | 5460 | 5650 |
| Shift of the line center due to the influence of foreign atoms | — | \(1 \cdot 10^{-4}\) Å per 1 mm of pressure of argon, present at a pressure of 5 mm | no impurities |
| Brightness of emission | — | good with 5 mm of argon, can operate at 0.25 mm | weak |
application, can be carried out with an accuracy depending on the sharpness and correctness with which the mark is made; in Lannoye’s review*) this accuracy is estimated at \(10^{-7}\)—\(5 \cdot 10^{-8}\). After Michelson compared the meter with the length of a light wave, proposals were made to adopt as the primary standard of length the light wave of the most monochromatic source; it was pointed out that such a standard would be the property of many laboratories and that it could not be damaged or destroyed, since it is reproduced, so to speak, by natural means. Lannoye points out that the last assertion is erroneous: reproduction of the standard—the light wave—is a difficult task, inaccessible to an ordinary laboratory. In addition, natural causes of the nonmonochromaticity of light (Doppler broadening of lines, natural width; the influence of collisions of atoms, the presence of isotopes and foreign substances) limit the accuracy of determining the length of a light wave and do not allow results better than with a line standard, at least with modern sources operating at not too low temperatures (as is known, Doppler broadening is proportional to the square root of the absolute temperature, and it plays the principal role). Therefore Lannoye sees no reason to abandon the meter standard and replace it by the length of a light wave. However, two objections may be raised against this: first, the meter standard can be destroyed; second, it can become deformed with time. The latter possibility makes rather unrealistic the naturally suggested proposal—to increase the reliability of comparison of length standards by choosing a longer standard (for when marks are compared, the absolute error does not depend on the length of the standard, while the relative error will decrease as the length increases). Of great interest are the data cited by Lannoye on modern sources of monochromatic light, collected in the table presented.
N. Malov