134: Dr. A. Oberbeck on the Friction 



believes rather that the processes observed by Plateau, espe- 

 cially the retardation of the motion of the magnetic needle at 

 the surface of some liquids, were occasioned partly by capil- 

 lary actions in consequence of the alteration of the form of the 

 free surface, partly by its being contaminated with foreign 

 substances. In particular, he assumes that the latter form a 

 thin elastic layer on the surface, which, on the motion of a 

 body in the surface, oppose a species of elastic action. In 

 his replies*, Plateau maintains his original view. Instead of 

 entering into a criticism of the conflicting opinions, it seemed 

 to me more important to examine the phenomena in question 

 afresh by a different method. This method had to fulfil the 

 following conditions: — 



(«) The effects of friction shall admit of being separated 

 from capillary actions at the surface, or from any surface - 

 elasticity which (according to Marangoni) may be present ; 



(b) The frictional resistances shall be expressed by nume- 

 rical values which can really be regarded as a measure for 

 them ; 



(c) These numerical values shall be comparable, both in the 

 interior and at the surface of the same liquid, and also in dif- 

 ferent liquids under like circumstances. 



It is easy to understand that the usual methods for the 

 determination of the coefficients of friction could not be em- 

 ployed here. In the flow of liquids through capillary tubes 

 the free surface does not at all come into consideration. 

 Moreover even the rotational oscillation of a disk is by no 

 means adapted for the investigation of the phenomena with 

 which we have here to do. If it be caused to oscillate at 

 different distances from the free surface, 0. E. Meyerf 

 has observed that the frictional resistances diminish on ap- 

 proaching it ; and this is exactly what was to be expected 

 according to the theory of these oscillations unfolded by 0. E. 

 Meyer | . According to it, by far the larger portion of the 

 action upon the disk depends on those liquid particles which 

 are vertically over or under the disk, and not on those which 

 lie in the same horizontal plane with it. If the layer of liquid 

 above the disk is very thin, it follows the oscillations of the 

 disk irrespective of the value of the coefficient of friction. 



Lastly, Plateau's method is well suited to demonstrate the 

 phenomenon qualitatively; but it does not permit us to 

 decide whether the longer time taken by the magnet on the 

 surface for its return to the position of equilibrium proceeds 

 from augmented friction or from other counteracting forces. 



* Bull, de VAcad. de Belg. [2] xxxiv. pp. 401-419 ; xlviii. pp. 106-128 

 (1880). 

 t Pogg. Ann. cxiii. p. 415 (1861). % Loc > cit - P- 62-67. 



