Parallel-plate Method Research Articles

The thermal conductivity of air by a parallel plate method

A measurement by a parallel plate method of the thermal conductivity of air was carried out by Hercus and Laby* in 1913-14. The value then obtained, though higher than previous values, is considerably lower than that given by the more recent work by the hot-wire method of Weber, Gregory and Archer, and Kannuluik and Martin,§ and the parallel plate method differs fundamentally in principle from the hot-wire method. The latter, as has been pointed out,∥ is subject to certain grave theoretical objections such as the difficulty of eliminating convection and at the same time allowing for the “Temperatursprung” at the wire. It is also open to question whether the geometrical measurements can be made with sufficient accuracy when a fine wire has to be place along the axis of an ordinary glass tube. It was therefore considered desirable to repeat the measurement by the parallel plate method with improvements in thermocouple technique and improved mechanical construction. Measurements by this method are carried out at atmospheric pressure with complete absence of convection, and it is interesting to note that Weber, who was not aware of the previous work, suggested the desirability of carrying out the measurement by a parallel plate method.¶ General Principle of Method . The general principle of the measurement is as described in the previous paper.* Three horizontal copper plates, fig. 1, are separated from each other by small vertical distances. The central plate is in two separate portions, the central disc B being thermally insulated from the guard-ring D. If B, D, and the uppermost plate A are maintained at the same constant temperature θ 2 while the lowest plate C is maintained at a constant lower temperature θ 1 , the heat supplied to the central disc B can escape only by conduction and radiation to D.

The thermal conductivity of air

When Miss Nelson and the writer prepared in 1929 an article for the 'International Critical Tables' on the thermal conductivity of gases, we found that the value for air had been measured by 19 observers and that the mean departure from the mean was 7%. Further the values obtained by the hot wire method by Weber, Gregory and Archer, and Schneider were higher than the value (5.40 X 10 -5 cal. cm. -1 sec. -1 deg. -1 ) which Hercus and the writer had found by a parallel plate method, and higher than 13 of the 14 determinations (including hot wire ones) made previous to 1918. In view of these facts it was desirable to repeat the parallel plate method and to obtain evidence as to whether or not there was a systematic difference between the two methods mentioned. The hot wire method, as used by the experimenters named, has the practical advantages measured, and it is convenient and simple. As carried out in the experiments referred to in which fine wires were used it has the disadvantages that the elimination of the convection effects is not attained with certainly, the temperature gradient in the gas is large (which introduces both theoretical and practical difficulties) and there is a temperature discontinuity at the surface of the wire which has to be determined. Hercus and Sutherland have nearly completed in this laboratory a measurement of the thermal conductivity of air with a parallel plate apparatus. This method has the inherent advantages that there are no convection currents in the horizontal lamina of gas used, that the temperature gradient may be made small, and the temperature disments now in progress radiation is eliminated by using the metal plates at two different separations and considerable improvements have been made us compared with the experiment of Hercus and in their measurement.

The Effect of Temperature on the Energy Distribution of Photoelectrons. I. Normal Energies

An experimental test has been made of DuBridge's theory of the distribution of normal energies of photoelectrons. With a modified parallel-plate method of analysis, photoelectrons were ejected from a strip of Mo foil which could be held at any desired temperature. Current-voltage curves for temperatures from 300\ifmmode^\circ\else\textdegree\fi{} to 965\ifmmode^\circ\else\textdegree\fi{}K show a good agreement with the theory. From the shifts required to fit the experimental and theoretical curves the values of ${V}_{max}$ at 0\ifmmode^\circ\else\textdegree\fi{}K can be determined. For any fixed temperature these values of ${V}_{m}$ fit the Einstein equation, and for a fixed wave-length they are independent of $T$, as required by the theory.

The thermal conductivity of water

The paper describes the precise measurement of the thermal conductivity of water in a range of temperatures extending from 7°C. to 60°C. The parallel-plate method has been employed, the apparatus being especially designed to cope with the difficulties which arise in the measurement of the conductivity of a volatile liquid. Two series of measurements were made, the plates in one apparatus having approximately three times the area of those in the other.

Societies and Academies

LONDON Physical Society, April 7.—A. H. BLATCHFORD: The diffraction of X-rays by liquid sulphur. The X-ray diffraction effects given by liquid sulphur at various temperatures between 130° C. and 260° C. indicate an unstable grouping of atoms of sulphur which becomes less pronounced with increased temperature, and changes in form gradually up to 220° C., when there is a sudden alteration corresponding to the change from the form Sλ; to Sμ. Raman and Ramanathan's theory of X-ray scattering by liquids is applicable to some extent at temperatures near the melting-point.—E. GWYNNE JONES: The hyper-fine structure of perturbed series. Rules governing such effects are derived. It is found that these rules are not analogous to those obtained from the analysis of multiplet structures.—A. J. BRADLEY and A. H. JAY: Quartz as a standard for accurate lattice-spacing measurements. A test has been made of the use of quartz as a standardising substance for very accurate lattice-spacing determinations. With copper-Ka radiation it gives a very good photograph with many sharp Ka doublets, which may be measured with accuracy. A redetermination of the axial ratio from an X-ray powder photograph gave c/a = 1-10002 ±0-00004, and this value was confirmed by X-ray measurements on several other crystals. Assuming Bergqvist's value for d100 (4246-02 X units) it was found that a = 4902-9 x., c = 5393-3 x. The values of d100 at 18° C. for different specimens all lay between 4245–95 and 4246–30. L. H. MARTIN and K. C. LANG: The thermal conductivity of water. The parallel-plate method was used over the range 7°-60° C., the apparatus being especially designed to cope with the difficulties which arise in the measurement of the conductivity of a volatile liquid. Two series of measurements were made, the plates in one apparatus having approximately three times the area of those in the other.

Absolute Wave-Lengths of the Copper and ChromiumK-Series

In the measurement of x-ray wave-lengths by ruled gratings two principal difficulties have been discussed which may account for the difference observed between the wave-lengths determined by this method and those determined by using crystal gratings. They are, the periodic error in the grating, and the geometrical divergence of the x-ray beam. It is now shown that the effect of the periodic error can be determined by a study of the intensities of the optical ghost lines. For a good quality optical grating it has been found that the error in x-ray wave-lengths due to the periodic error in the grating is thus of no importance. By using a suitable disposition of apparatus the effect of the geometrical divergence of the x-ray beam can be made as small as desired. Thus it is concluded that ruled gratings can be used for precise wave-length measurements of x-ray spectra.In the present experiment the two parallel plate method has been used for determining the angles of incidence and diffraction. Five glass gratings of different grating spaces and ruled on two ruling engines have been used. The results from the various gratings on the same wave-length have agreed satisfactorily. No consistent variations of any type were observed. The final results from 172 sets of plates are given in the following tables. From these results the true grating space of a calcite crystal is $d=3.0359\ifmmode\pm\else\textpm\fi{}0.0003$ A. Using this value of the grating constant, Planck's constant as determined by Duane, Palmer and Yeh is, $h=6.573\ifmmode\pm\else\textpm\fi{}0.007\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}27}$ erg\ifmmode\cdot\else\textperiodcentered\fi{}sec. $\frac{e}{m}$ can be determined from the dispersion of x-rays by using the absolute wave-length of an x-ray spectrum line. The mean of the values of the dispersion as given by Stauss and Larsson gives $\frac{e}{m}=1.769\ifmmode\times\else\texttimes\fi{}{10}^{7}$ e.m.u.${\mathrm{g}}^{\ensuremath{-}1}$. The values of these constants are independent of any imperfection in the crystal. If the crystal lattice is assumed to be perfect we then have Avogadro's number, $N=6.019\ifmmode\times\else\texttimes\fi{}{10}^{23}$ mol. per mole, and the charge on the electron $e=4.806\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ e.s.u. Using this value of $e$, and $d$ as above, we find Planck's constant $h=6.623\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}27}$ erg\ifmmode\cdot\else\textperiodcentered\fi{}sec.