On Friction heiiveeii Surfaces moving at Low Speeds, 309 



second, aud hnve found that in certain cases there is decided in- 

 crease in the coefficient of friction as the velocity diminishes. 



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 supported in bearings which consisted of rectangular notches 

 cut in pieces of the material whose friction against steel was to 

 be measured. The disk was caused to revolve and then left to 

 itself, M'hen it came to rest in consequence of the friction on the 

 ends of the axle. The rate of retardation was found as f ollo^^■s : — ■ 

 A strip of paper 24 inches broad was stretched round the periphery 

 of the disk; and a pendulum was caused to swing across this paper 

 in a plane perpendicular to that of the disk. On the pendulum was 

 fastened a line glass siphon, one end of which dipped into a box 

 containing ink, whilst the other stood at a short distance from the 

 paper strip, across which it was carried as the pendulum oscillated. 

 By keeping the ink-box strongly electrified, ink was deposited on the 

 paper by tlie point of the siphon in a rapid succession of fine spots. 

 By this means, without the introduction of any new source of 

 friction, a permanent record ^^as made of the resultant motion of 

 the pendulum and the revolving disk. This frictionless method of 

 recording was designed by Sir "William Thomson for telegraphic 

 purposes, and is employed in his siphon recorder. From the curve 

 drawn in this way it was easy to determine the rate of retardation 

 of the disk (and therefore the friction) corresponding to various 

 velocities of the rubbing surfaces. The lowest velocity for which 

 the determinations were definite was about 0*0002 foot per second; 

 and the highest velocity to which the experiments extended Avas 

 0*01 foot per second. The surfaces examined were steel on steel, 

 steel on brass, steel on agate, steel on beech, and steel on green- 

 heart — in each case under the three conditions, dry, oiled, and wet 

 with water. In the cases steel on beech oiled or wet with water, and 

 steel on greenheart oiled or wet with AA'ater, the coefticient of fric- 

 tion increased as the velocity diminished between the two limits 

 given above, the increase amounting to about twenty per cent, 

 of the lower value. It appeared that at the higher limit of 

 velocity there was little further tendency to chauge in the co- 

 efficient ; but it is impossible to say how much additional chauge 

 might take place between the lo"\Aer limit of the velocity and rest. 

 In the case of steel on agate wet \Aith water there was a similar 

 but much less marked increase of friction as the velocity de- 

 creased ; and in the case of steel on steel oiled there Mas a slight 

 and somewhat uncertain change of the opposite character — that 

 is, a decrease of friction as the velocit}^ decreased. This case, 

 however, would require further examination. In all other cases the 

 friction seemed to be perfectly constant and independent of the 

 velocity. Out of all the sets of circumstances investigated, the 

 only ones in which there was a large difference between the static 

 and kinetic values of the coefficient of friction were those in which 

 a decided increase Avas observed in the kinetic value as the speed 



