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:—
1. "When the oil has dried up, the friction of a steel pivot in brass is actually less than in agate.
2. "A greater diminution of friction, by the application of oil, is effected when steel is used with steel, than where steel is used with brass or agate; although the fluid friction is probably equal in the three cases, when oil is used.
3. "With a perfect, non-drying, non-oxidizing lubricant, steel bearings for pivots would be preferable to brass bearings. Hence, with anything short of an approximately perfect oil, the brass is most serviceable.
4. "Brass pivot holes are much less affected by the drying of the oil than agate holes would be; and, in the absence of experiment, we must assume that this would be the case with ruby or other jewels.
5. "When the oil is perfectly fresh, agate and steel have a very low coefficient of friction."
How much these results would be altered by the use of a disk of such weight, and pivots of such proportionate size, as to meet the actual requirements in horology, remains to be ascertained.
Certainly the experiments of Jenkin, are not applicable to the pivots of a watch, as stated by E. Rigg; especially are they not applicable to the friction of pivots in the escapement, where the laws of fluid friction are more nearly applicable; and when it is remembered that the weight of the disk was 86 pounds, and the pivots .25 c. m. in diameter, (or about the size of pivot of a large barrel arbor,) it is evident that the solid friction produced was much in excess of that produced in even the heavy part of the train of a watch.
Furthermore, even Jenkin and Ewing, in their paper, state "that, owing to the very great intensity of the pressure on the small bearing surfaces of the axle, the lubricants must have been to a great extent forced out." In a properly made watch, with a good lubricant, this does not occur.
But there can be no doubt that if the apparatus above described were so constructed as to meet the actual conditions in time recording instruments, very valuable data could be thereby secured. This could be done by reducing the weight of the disk, so as to make the weight bear the proper relation to the size of the pivots.