FRICTION. In Mechanics, the resist ance produced by the rubbing of the sur faces of two solid bodies against each other. If the surfaces of bodies were perfectly smooth and polished, they would slide along one another without suffering any resistance from their con tact, and all the simple relations between power and resistance determined by the ory in respect of the different machines would hold good without any modifica tion whatever. But this state of perfect polish never exists. The surfaces of all bodies with which we are acquainted, even when most carefully polished, re tain a greater or less degree of asperity, which prevents them from sliding over one another without impediment ; and in many cases the resistance thus created amounts to a large proportion of the whole resistance to be overcome. In or der, therefore, to ascertain the real value of the effect of powers applied to machin ery, it is neccsaary to determine the amount of the friction, and to add this new resistance to that which is given by the theory of mechanics.
The determination of the laws of fric tion, and its amount with respect to par ticular substances, have occupied the at tention of many experimental philoso phers and mathematicians, as Amontons, Euler, Desaguliers, Vince, &c. ; but the first complete set of experiments on the the subject was made by Coulomb about the year 1780. His results, though they have been partly modified by subsequent experiments, throw much light upon the subject, and are of great value to the practical engineer.
There are two modes by which the na ture and operation of friction may be as certained. The first is very simple, and consists in merely placing a heavy body on a horizontal plane, and elevating the end of the plane till the body begins to slide. When this motion commences, it is evident that the force of gravity just begins to exceed the resistance occa sioned by the friction ; and as the gravi ty is known from the weight of the body, and the inclination of the plane, we have thus the means of comparing the friction with a given force.
But this method is liable to some un certainty. Most bodies, after having been in contact for some time, require a greater force to originate than to keep up progressive motion ; but it is obvious that the inclination of the plane of de scent marks only the initial obstruction. Coulomb accordingly adopted a different mode of proceeding. His general me
thod was to draw a sort of loaded sledge along a horizontal bench, by means of weights placed in a dish attached to the sledge by a cord passing over a pulley. The sledge was mounted on sliders of the substance on which the experiments were to be made ; and the corresponding slips of the same or a different substance placed upon the sliders on the bench. This apparatus has been called a tribome ter. The following are some of the re suits which were obtained.
Assuming the pressure as equal to 100 parts, the friction of oak against fir was 66 in the direction of the fibres, but amounted only to 16 when moved with the velocity of a foot each second ,• the friction of oak against oak in the direc tion of the fibres was 43, and across them only 27, the effect being still reduced by motion to 10; the friction of fir against fir in the direction of the fibres was 56, which sunk to seventeen during motion ; the friction of elm against elm in the di rection of the fibres was 46, and reduced by motion to 10. On the other hand, the friction of copper upon oak, length wise, was 8 at the commencement of the motion, but increased to 18 when the velocity was a foot in a second • the fric tion of iron upon oak with the initial velocity was 11, and was increased by the motion to 18. But the mutual friction of metals appeared in general to be scarce ly, if at all, affected by motion. In these experiments no unguents were used.
Where metals rub against wood, it is necessary that the two bodies continue longer in contact, in order that the fric tion may acquire its maximum. In the case of iron against wood at least 4 or 5 hours must elapse before the momentary increase of friction disappears ; whereas in the case of wood against wood a sin gle minute was sufficient. But the re sistance appears to increase by contact, though less sensibly, even for several days. The application of grease to the surfaces of wood produces a similar ef fect, and the resistance does not attain its maximum till after a very considerable time. At the end of 5 or six days the resistance is perhaps 14 times greater than it was at the first instant, if the sur face of contact is considerable in respect of the pressure ; but when the surface is small, the friction reaches its maximum much more quickly.