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T,T OYD R . T ~ o n i . 4 AND ~ FRANCOTS OLMER [ C O N rRIBUTION FROX T H E CHEMICAL
LABORAIORY, UNIVERSITY
Vol. 65 O F MISSOURI]
The Accommodation Coefficients of He, Ne, A, H,, D,, 0,, CO,, and Hg on Platinum as a Function of Temperature BY LLOYD B. THOMAS AND FRANCOIS OLMER
Introduction
We have described in a preceding paper' an apparatus similar to that used by other investigaWhen one attempts to apply the Pirani type tors for measuring accommodation coefficients pressure gage to the problem of following quantitatively the course of a reaction involving gases and applied i t to a determination for mercury vaa t low pressure, he must have a t hand a knowl- por on platinum. We have since applied this edge of the free molecule heat conductivity for apparatus, involving 0.0025-cm. potential leads each gas, consumed or produced in the reaction, on a 0.01-cni. main filament, in a preliminary set from the particular filament surface used in the of measurements of the accommodation coeffigage. This low pressure heat conductivity may cients of hydrogen, helium, oxygen and argon in be most conveniently expressed as the product of a pressure range of 0.01 to 0.03 mm. The results the theoretical heat conductivity (involving the are described below with reference to the correheat capacity, molecular weight aiid temperature sponding curves of Fig. 4. The curves for hyof each gas) and the accoinmodation coefficient drogen and helium meet those of Fig. 4 tangenof each gas upon the particular filament surface tially in the region of AT = 300°, are concave material. This latter expresses the average tern- upward, and have intercepts on the AT = 0 axis perature increment of the gas molecules, after en- about 4092 higher than those of Fig. 4. The gagement with the surface, as a fraction of the curves for oxygen and argon meet those of Fig. 4 temperature difference between the filament and in the region of AT = 300°, are concave upward, the incident molecules. Upon examination of and would give extrapolated intercepts a t AT = values of the accommodation coefficients of vari- 0 of approximately unity. Although the diEous gases on platinum available in the literature culty of obtaining consistent values of the accorni t was evident that the values were not consistent rriodation coefficient increases as AT becomes enough among the various investigators to be smaller, repeated attempts were made to apusable with any degree of certainty. The available proach the limiting value as AT -+ 0 with results values are listed and discussed later in the paper. consistently suggesting a value of unity for argon We have found, aside from a number of isolated and oxygen. The truth of this result, of course, measurements on one or two gases, but two sets was doubted; so a detailed consideration of erof accommodation coefficient measurements on a rors in the method was undertaken, especially sizable number of gases on platinum in the low errors due to the temperature distribution in the presstire range and these date from 1910 and 1915 filament arid leads. These considerations are outbefore modern vacuum technique was available. lined below since they apply to apparatus of other 'The relative values of accommodation coefficients investigators and since they show the need for o f these sets of gases deterIliined on the same the conditio11 of uniform temperature throughout filanient should be more significant than isolated the filament which i t is hoped is achieved by the values. .llso, from the standpoint of application apparatus described subsequently. I'he fdkJWing differential equation, which apt u the Pirani gage. the few investigatioris in which proxuria k l y rrpreserits the hedt produced electrithe coefficients were determined a t it series of terncally aiid the heat gained or kJSt by nietallic conIm-atures show a considera1)Ie variatioii of the valtluctioii ctloi~g the filament, the heat radiated ues with temperature so one could not use values from tlie surface, and heat conducted by the gas i)k coefficients a t teniperaturcs other than those a t froni tiic w r f x c for an infinitesimal segment of which they :ire detenniried. 'The origi~idirnmc.the filLtlli
