44. Friction Between Surfaces Moving at Very Slow Speed, has been investigated by Fleming Jenkin and J. A. Ewing. A contrivance, which would be very excellent with some improvement, for the determination of the amount of friction under such conditions, is given in a paper[8] read before the Royal Society of London.
The arrangement employed by them was composed of a cast iron disk two feet in diameter and weighing 86 pounds. This disk, being turned true on its circumference, was supported by a spindle terminating in pivots 0.25 C. M. in diameter, the pivots resting in small rectangular bearings composed of the material the friction of which with steel is to be determined.
A tracing of ink was produced on a strip of paper which surrounded the disk, the ink being supplied by a pen actuated electrically by a pendulum, as in the syphon recorder.
As the traces thus left on the paper were produced without in any way interfering with the freedom of motion of the disk, they afforded a means of determining the velocity of rotation.
The relative velocities of the pivot to the bearing surfaces varied from .006 C. M. to 0.3 C. M. per second, being the velocities met with in the various parts of time keeping devices.
Experiments were made with the bearing surfaces successively in three different conditions: viz. 1, dry; 2, wet with water; and 3, wet with oil; and gave the following results:
TABLE I.
| SURFACES. | COEFFICIENT OF FRICTION. | |||
| JOURNAL. | BEARING. | DRY. | WATER. | OIL. |
| Steel | Steel | 0.351 | 0.208 | 0.118 |
| " | Brass | 0.195 | 0.105 | 0.146 |
| " | Polished Agate | 0.200 | 0.166 | 0.107 |
Several facts of great interest to the horologist are here shown. [9] Edward Rigg has this to say in regard to the apparatus of Jenkin and Ewing. "The friction, then, is true sliding friction without any rolling, and it will be evident that if the bearing were a circular hole just large enough to admit the pivot freely, the character of the friction would be in no way changed. In both a watch and clock the pivots are pressed against the sides of the pivot holes, either by the motive force or by gravity. There is no rolling round the pivot holes, so that the friction is all of the first kind. Jenkin's experiments are, then, strictly applicable to the case of pivots,[10] and they constitute, so far as I am aware, the first scientific determination of the friction that occurs in time-keepers, and even in these experiments, the pressure, due to the weight of 86 pounds, is evidently too great, and thus too little regard is paid to the influence of adhesion."
E. Rigg further states that, reverting to the preceding table, we notice the following points of interest:—