Dispersion and Absorption of Water and Ethyl Alcohol in the Region of Short Electric Waves.

Willy Möbius. Über die Dispersion von Wasser u. Äthylalkohol zwischen 7 und 35 mm. Wellenlänge und Vorversuche zur Verwendung nach kürzerer elektrischer Wellen. Annalen der Physik. B. 62. p. 293, 1920.

The refractive index of water in the region of short electric waves, as was shown already by the earlier investigations of Lamp and later ones, is close to the square root of the dielectric constant and is about 8.

The refractive index of water in the region of long heat waves, 0.3 mm, is only slightly greater than in the visible spectrum. This leads one to assume, in the intermediate region, a band of anomalous dispersion, the identification of which the author sets as the task of his investigation. The construction of the vibrator does not differ from earlier ones (Righi, Lebedev).

The receiver was always a comparatively long wire, not tuned to resonance and possessing, owing to the inclusion of a thermoelement with a resistance of 250–400 Ω, large damping; such a system makes it possible to use one and the same receiver for different wavelengths.

The measurement of the wavelength is carried out by means of Boltzmann mirrors or a quartz interferometer (after Rubens and Hollnagel).

On the interference curves the presence of a large number of incoming waves shorter than the fundamental wavelength of the vibrator and superposed upon it was detected. Waves of 0.1; 0.11; 0.17; 0.25; 0.30; 0.50; 0.70; 1.0; 1.1; 1.2; 1.7; 1.8; 2.0; 2.5; 3.0; 3.8; 4.0; 5.0 mm were observed; waves of 2.0, 2.5, and 3.0 mm occurred most often. The incoming short waves have less damping than the fundamental one and are very changeable, which makes working with them and isolating them difficult. Some vibrators give almost pure short waves, while the fundamental wave is not noticeable.

The occurrence of short waves may be explained partly by the presence of overtones of the vibrator; shorter and variable waves may arise from oscillations occurring in metallic particles torn off by the spark and, finally, it is possible to suppose oscillations of positive metal ions about negative ones; in this last case calculation gives the emitted wavelength:

\[ \lambda = 0{,}005472 \frac{A}{\sqrt{d}} \ \text{mm}, \]

where \(A\) is the atomic weight, \(d\) the density of the metal, which for platinum gives:

\[ \lambda = 0{,}232 \ \text{mm}. \]

These considerations make it possible to suppose that different metals can give different constant wavelengths.

The isolation of short waves was achieved in one experiment by means of a mirror covered with strips of tinfoil arranged in the form of a regular grating, similar to a crystalline one; the length of a strip was 0.8 mm, the width 0.2 mm, the spacing in width 0.8 mm, and in length 0.2 mm. With the proper wavelength of the vibrator being 18.8 mm, a wave \(\lambda = 3.0\) mm was isolated; but after two days the same vibrator already gave an entirely different wave, which shows the difficulty of applying short waves to systematic investigations. For waves in the interval from 35 to 7 mm, measurements were made of absorption, dispersion, and reflection independently of one another. The absorption coefficient was measured from the change in the intensity of oscillations when passing through a definite layer of liquid poured into a flat vessel; diffraction phenomena for such short waves can be quite neglected. The reflection coefficient was measured compar-

...by measuring the intensity of the reflected and direct ray. The refractive index was measured from the deviation in an ebonite prism filled with liquid.

From the absorption coefficients \(k\) and reflection coefficients \(R\), it is possible to calculate the refractive index by the optical formula:

\[ n=b\cos\varphi+\sqrt{(b^2-1)\cos^2\varphi-\eta^2} \]

where

\[ b=\frac{1+R}{1-R}, \]

and \(\varphi\) is the angle of incidence.

Thus, the curves show the absorption and, independently of it, the dispersion measured directly and calculated from \(R\) and \(k\). The absorption curve has a sharp maximum at \(\lambda=22\) mm. At the same wavelength the refractive index has a sharp minimum, \(n=5\); at \(\lambda=35\) mm, \(n=7.6\); at larger wavelengths, as is evident from many measurements by other authors, \(n\) reaches a value of 9; toward shorter waves \(n\) increases to 7 at \(\lambda=10\) mm, and then again begins to fall; as the single measurement at \(\lambda=3\) mm showed, \(n=4.3\), which fits well with the continuation of the author’s curve, but disagrees with Lampa’s old measurements, who found in this region \(n\) about 9.

For ethyl alcohol, a fall of absorption into the region of short waves was found; the refractive index falls from 3.5 (\(\lambda=60\) mm) to 2 (\(\lambda=7\) mm).

The absorption band found by the author at \(\lambda=22\) mm allows him to propose a new method for isolating short incident waves; namely, by giving the fundamental wave of the vibrator a length of 22 mm and placing a layer of water in front of it, it is possible completely to absorb the fundamental wave and to isolate the short incident waves.

S. Rzhevkin.