hogg. — friction and force due to transpiration. 117 



Problem in Detail. 

 Introduction. 



Aside from the interest which attaches itself to an investigation of 

 the law relating viscosity to pressure at low pressures, the advantage 

 of being able to measure gas pressure by measuring the friction of the 

 gas on a pendulum is apparent. By this method the state of the gas 

 remains unchanged during the time the measurement is in progress, 

 while by the method of the McLeod gauge the gas must be compressed 

 in order that the pressure may be measured. It is, however, beyond 

 doubt very important that in establishing a method of gas pressure 

 measurement by friction, or otherwise, a second method should if 

 possible be obtained to serve as a check upon the one which we may 

 consider most convenient and therefore the most desirable. Though 

 the investigation of the law relating pressure in a gas to transpiration 

 force when the transpiration space is of special form is interesting and 

 important for its own sake, yet additional interest and importance is 

 imparted to it when the possibility of making its results serve as the 

 desired check upon the proposed friction method of measuring gas 

 pressure is considered. 



Thus, in so far as the object of the present investigation is to test or 

 replace the McLeod gauge, the function of what has been called the 

 transpiration apparatus is to serve as a check upon the results of 

 pressure measurement obtained by the viscosity apparatus. As will 

 appear in the sequel, the problem to be solved with the transpiration 

 apparatus is that of investigating the law of gas action on a radiometer 

 vane of special form surrounded by a containing vessel whose parts are 

 symmetrically placed with respect to the vane. 



That at least another attempt should be made to decide the question 

 as to how much reliance can be placed on the McLeod gauge for meas- 

 urement of pressures below 0. 1 cm. is very forcibly brought home to one 

 who examines the conflicting evidence of high authority. 



The attempts to investigate the behavior of the gauge when the vacua 

 are high, have as a rule been made, not with the avowed intention of 

 testing the gauge, but with the object of testing the validity of Boyle's 

 law at various pressures, any departure from which for any gas would 

 at once limit the use of the gauge for that particular substance. The 

 method used was to enclose a certain quantity of gas, measure accu- 

 rately its volume and its pressure, calculate /? ?; ; change this volume, 

 and therefore pressure ; measure again, and so on. The various values 

 oi pv thus obtained should be the same, that Boyle's law may be 

 obeyed, and hence that the principle of the gauge may be sound. 



