1877.] On Friction between Surfaces at Low Speeds. 93 



A comparison of the results here obtained with those detailed in Part 

 II. of these researches shows that the phenomena produced by suitable 

 disposition of the Leyden battery coincide with those produced by the 

 induction-coil. With the coil it was found that (1) for a given electro- 

 motive force the column of striae was shorter the larger the battery- 

 surface or strength of current used ; (2) that the proper motion, when 

 directed, as usual, towards the positive terminal, was more rapid the 

 greater the electromotive force employed. With the Leyden battery it 

 was found that (1) in order to maintain the same length of column with 

 an increased surface, the charge must be increased in a larger proportion 

 than the surface ; and (2) it was noticed that the striae, which when the 

 tension was low were distinct and well separated, became more blurred 

 as the tension rose, until they sometimes were blended into an apparently 

 unbroken column of light. The presence, however, of the negative glow 

 still showed that the true jar-discharge had not yet been reached. 



II. " On Friction between Surfaces moving at Low Speeds/ 5 By 

 Fleeming Jenkix, F.R.SS. L. & E., Professor of Engineering 

 in the University of Edinburgh, and J. A. Ewixg. Received 

 March 8, 1877. 



(Abstract.) 



The common belief regarding friction, which is based on the researches 

 of Coulomb and Morin, is that between surfaces in motion the friction 

 is independent of the velocity, but that the force required to start the 

 sliding is (in some cases at least) greater than the force required to over- 

 come friction during motion; in other words, the static coefficient is 

 usually considered to be greater than the kinetic. It occurred to the 

 authors that there might possibly be continuity between the two kinds of 

 friction, instead of an abrupt change at the instant in which motion 

 begins. "We should thus expect that when the relative motion of the sur- 

 faces is very slow there will be a gradual increase of friction as the velocity 

 diminishes. Whether any such increase takes place at very low speeds 

 is left an open question by the experiments of Coulomb and Morin, whose 

 methods did not enable definite measurements of the friction to be made 

 when the velocity was exceedingly small. The authors have succeeded 

 in measuring the friction between surfaces moving with as low a velocity 

 as one five- thousandth of a foot per second, and have found that in cer- 

 tain cases there is decided increase in the coefficient of friction as the 

 velocity di min ishes. 



The apparatus made use of consisted of a cast-iron disk 2 feet in 

 diameter and weighing 86*2 lbs., supported on a steel axle whose ends 

 were less than one tenth of an inch in diameter. These ends were sup- 

 ported in bearings which consisted of rectangular notches cut in pieces 



