Physics. — "The Injiuence of Rotation on the Sensitivenesf! and 

 the Accuracy of a Pressure Balaiice." (Twelftli communication 

 of results obtained in researches made by the aid of the 

 Van der Waals fund). By A. Michels. (Communicated by Prof. 

 P. Zeeman). 



(Comnaunicated at the meeting of October 27, 1923). 



For the accurate measurement of great pressures methods are 

 now of general application, based on the use of the so-called 

 Amagat cylinder. In all these methods the force is studied exerted 

 by a liquid under pressure on a piston of known diameter. The 

 elaboration of this fundamental idea has given rise to different types 

 of pressure balances, as those of Wagner, Stückradt, Schaffer und 



BüDENBERG, and HOLBORN '). 



In order to reach an accuracy as great as possible it is necessary 

 to reduce the friclional forces between the piston and the wall of 

 the hole to a minimum. In this respect Wiebe already obtained 

 good results by tapping the wall of his apparatus, with a hammer. 

 Of late a rotation of the piston has pretty generally been applied, 

 though HoLBOHN ') considers a movement to and fro preferable. 



The causes why these operations have such an influe^nce, are 

 only imperfectly known as yet. Klein (loc. cit.) tries, indeed, to 

 give a solution of the effect of rotation, but does not succeed. 



The purpose of this investigation is to find a solution, and at 

 the same time to determine the circumstances under which the 

 greatest effect is reached. 



As there is no room here for an extensive discussion of our 

 results, we shall restrict ourselves in what follows to a brief com- 

 munication, referring for a fuller treatment to "Annalen der Physik" 

 Bd. 72, 1923, p. 285—320. 



It was tried to work theoretically in the direction indicated by the 

 recent theory of bearings lubricated all round '). For when the piston 

 revolves in a cylindrical hole, liquid being continually supplied from 

 below, there must certainly be an analogy between the influences 

 of friction to which our piston is subjected and those exerted on 

 an ordinary axle resting in a bearing block. 



M I refer for the ciiffeient types to Klein. G. Untersuchung und Kritik von 

 Hochdruckmesser Diss. Berlin 1909. 



•i) Ann. d. Physik 1915, p. 1087. 



'I SoMMERFELD. Zeitschf. füi' Math, und Physik 1904, Gümbel. Das piobleni der 

 Lagen-eibung Jahresb. d. Schiffbautechn. Gesellsch. 1917. 



