While fluid friction acts as a retarding force in mechanism it converts the mechanical energy required to produce it into its heat equivalent, thus raising the temperature of the mass in a greater or lesser degree.
The resisting property which thus effects this conversion, and which is the cause of fluid friction, is called viceosity.
It is thus apparent that a variation of the viceosity of the oil used on a watch would cause a variation of fluid friction and consequently a variation of the effort (11), and would seriously interfere with the rate of the watch. This will be discussed (84) more thoroughly in another paragraph.
40. The Laws of Fluid Friction are:
1. Fluid friction is independent of the pressure between the masses in contact.
2. Fluid friction is directly proportional to the surfaces between which it occurs.
3. This resistance is proportional to the square of the relative velocity at moderate and high speeds, and to the velocity nearly at very low speeds.
4. It is independent of the nature of the surfaces of the solid against which the stream may flow, but it is dependent to some extent upon the degree of roughness of those surfaces.
5. It is proportional to the density of the fluid and is related in some way to its viscosity.
41. The Compound Friction of Lubricated Surfaces, as Thurston terms it, or friction due to the action of surfaces of solids partly separated by a fluid, is observed in all cases in which the rubbing surfaces are lubricated. The solids, in such instances, though partly supported by the layer of lubricant which is retained in place by adhesion (21) and cohesion (20), usually rub on each other more or less, as they are usually not completely separated by the liquid film interposed between them.