A simple machine, of great utility in cases where an immense pressure and little motion are required. The wedge may be considered a modification of the inclined plane, to which in many cases it is strictly analogous, differing only in the circumstance that the body to be moved is drawn along the surface of the plane; but in the wedge the plane is made to move by percussion beneath the body to be raised, or between the surfaces to be separated. Wedges are frequently employed for splitting masses of timber or stone; ships are raised in docks by wedges driven under their keels. Sometimes they have been employed to restore a declining edifice to the perpendicular position. In the annexed cut the wedge a c b is employed in cleaving wood, and its mechanical power is estimated by the proportion of a b to d c. This is sometimes differently stated, and it is difficult to state positively what is the exact power obtained by the use of the wedge, as it is generally driven by blows of a mallet or hammer; there can, however, be no doubt that the penetrating power is increased by increasing the length d c, in proportion to the breadth a b.
The wedge, in part, owes its value to a quality which, in most machines, is a diminution of their effect, i. e. the friction that arises between it and the substance it divides. Were it not for the immense friction which obtains in the use of the wedge, it would recede to its original position, between the successive blows, and thus no progress would be made. Instead of this, however, we find the pressure and adhesion of the surfaces prevent the recoil, and thus a succession of slight blows effect a result which previously might have been supposed beyond human power to realize. All cutting and piercing instruments, as knives, chisels, razors, nails, pins, etc, may be considered as wedges. The angle of the wedge, in these cases, is more or less acute, according to the purposes to which it is to be applied. The mechanical power of the wedge is of course increased by diminishing the angle, but as this diminishes the strength of the instrument, there is a practical limit to this increase of power.
In tools intended for cutting wood the angle is generally about 30°. For iron it is from 50° to 60°; and for brass from 80o to 90o. Tools which act by pressure may be made more acute than those which are drawn by percussion, and in general the softer the substance to be divided, and the less the power required to act upon it, the more acute may be the construction of the wedge.