FRICTION
In making such calculations, however, there is an additional element which the engineer must consider, but which we have hitherto disregarded. In all methods of transmission of power, and indeed in all cases of the contact of one substance with another, there is an element of loss through friction. This is due to the fact that no substance is smooth except in a relative sense. Even the most highly polished glass or steel, when viewed under the microscope, presents a surface covered with indentations and rugosities. This granular surface of even seemingly smooth objects, is easily visualized through the analogy of numberless substances that are visibly rough. Yet the vast practical importance of this roughness is seldom considered by the casual observer. In point of fact, were it not for the roughened surface of all materials with which we come in contact, it would be impossible for any animal or man to walk, nor could we hold anything in our hands. Anyone who has attempted to handle a fish, particularly an eel, fresh from the water, will recall the difficulty with which its slippery surface was held; but it may not occur to everyone who has had this experience that all other objects would similarly slip from the hand, had their surfaces a similar smoothness. The slippery character of the eel is, of course, due in large part to the relatively smooth surface of its skin, but partly also to the lubricant with which it is covered. Any substance may be rendered somewhat smoother by proper lubrication; it is necessary, however, that the lubricant should be something which is not absorbed by the substance. Thus, wood is given increased friction by being moistened with oil, but, on the other hand, is made slippery if covered with graphite, soap, or any other fatty substances that it does not absorb.
Recalling the more or less roughened surface of all objects, the source of friction is readily understood. It depends upon the actual jutting of the roughened surfaces, one upon the other. It virtually constitutes a force acting in opposition to the motion of any two surfaces upon each other. As between any different materials, under given conditions, it varies with the pressure, in a definite and measurable rate, which is spoken of as the coefficient of friction for the particular substances. It is very much greater where the two substances slide over one another than where the one rolls upon the other, as in the case of the wheel. The latter illustrates what is called rolling friction, and in practical mechanics it is used constantly to decrease the loss—as, for example, in the wheels of wagons and cars. The use of lubricants to decrease friction is equally familiar. Without them, as everyone knows, it would be impossible to run any wheel continuously upon an axle at high speed for more than a very brief period, owing to the great heat developed through friction. Friction is indeed a perpetual antagonist of the mechanician, and we shall see endless illustrations of the methods he employs to minimize its influence. On the other hand, we must recall that were it rendered absolutely nil, his machinery would all be useless. The car wheel, for example, would revolve indefinitely without stirring the train, were there absolutely no friction between it and the rail.