L 201 j 



XXI. Static Friction.— II. By W. B. Hardy *. 



A. Chemical Constitution and the Lubrication 

 of Bismuth. 



^■^HESE experiments are a continuation of earlier work 

 X on the static friction of glass faces (Phil. Mag. ser. 6, 

 vol. xxxviii. p. 32 (1919)). A slider having a curved surface 

 was applied to a plane surface, both slider and plate being 

 of bismuth f. This metal was chosen because it is highly 

 crystalline and at the same time takes a high degree of 

 polish. It therefore offered unusual facilities for com- 

 paring the friction of the amorphous slate of the metal 

 found on a burnished face % with that of the crystalline 

 state. 



One of the faces being curved, contact was over an area 

 defined by the weight and the elasticity of the material. 

 The extent of this area is unknown, and the normal pres- 

 sure over it not uniform. There are therefore unknown 

 quantities which make it impossible to express the static 

 friction in terms of a normal pressure. The observations 

 of Burgess § show that nothing would be gained in this 

 respect by employing two plane surfaces. The best ob- 

 tainable surfaces touch only at points, and if of glass, 

 show Newtonian colours. 



The tractive force was applied slowly, and in such a way 

 as to make the slider rock forwards. The static friction 

 therefore is that between freshly applied faces. The 

 method is apt to give undue importance to viscosity, and 

 care is needed to distinguish between transitory effects due 

 to viscosity and true lubrication (see the earlier paper). 



Passing from glass to bismuth faces, one enters another 

 world. Meticulous care perfected by practice is needed 

 to secure clean glass faces, and, once secured, they readily 

 contaminate by the spreading of a film of matter from 

 solids with which they are in contact or by condensation 

 from the air. The edges of a plate oppose the spreading 

 of most if not all fluids, and it is this interesting property of 

 edges which renders exact. work with glass possible. 



Bismuth, on the other hand, can be cleaned by simply 

 rubbing the face with wash-leather, and the clean plate 



* Communicated by the Author. 



t I am indebted to mv friend Mr. Hevcoek for a specimen. 



% Beilbv, Proc. Roy. Soc. lxxii., p. 218 (1993). 



§ Proc.'Roy. Soc. A. lxxxvL, p. 25 (1911). 



Phil. Mag. S. G. Vol. 40. No. 236. Aug. 1920. P 



