Brush (from Fr. brosse, which, like the English word, means both the undergrowth of a wood and the instrument; if the word in both these meanings is ultimately the same, then the origin is from a bundle of brushwood used as a brush or broom, but this is historically doubtful, and others connect it with the Ger. Borste, bristle), an instrument for removing dust or dirt from surfaces or for applying paint, whitewash, etc., composed of a tuft or tufts of some fibrous or flexible material secured to a solid basis or stock. Brushes made of the twigs of trees like the birch and provided with long handles are often called brooms, and the same term is applied to some brushes used in the household for removing dust (e.g. carpet-broom, whisk-broom) but not to those used for applying paint. Among the numerous materials employed for the manufacture of brushes of various kinds are feathers, pig's bristles, the hair of certain animals, whalebone, rubber, split-cane, broom-corn (a variety of sorghum) and coir.

Brushes are of two kinds, simple and compound. The former consist of but one tuft, as hair pencils and painters' tools. The latter have more than one tuft. Brushes with the tufts placed side by side on flat boards, as plasterers' brushes, are called stock-brushes. The single tuft brushes, or pencils for artists, are made of the hair of the camel, badger, goat and other animals for the smaller kind, and pig's bristles for the larger. The hairs for pencils are carefully arranged so as to form a point in the centre, and, when tied together, are passed into the wide end of the quill or metal tube and drawn out at the other end to the extent required. The small ends of the quills, having been previously moistened, contract as they dry and bind the hair. A similar effect is produced with metal tubes by compression. Compound brushes are - first, set or pan-work; second, drawn-work. Of the former, an example is the common house-broom, into the stock of which holes are drilled of the size wanted. The necessary quantity of bristles, hair, or fibre to fill each hole being collected together, the thick ends are dipped into molten cement chiefly composed of pitch, bound round with thread, dipped again, and then set into a hole of the stock with a peculiar twisting motion.

In drawn-brushes, of which those for shoes, teeth, nails and clothes are examples, the holes are more neatly bored, and have smaller ones at the top communicating with the back of the brush, through which a bight or loop of wire passes from the back of the stock. Half the number of hairs of fibres needed for the tufts to fill the holes are passed into the bight of the wire, which is then pulled smartly so as to double the hairs and force them into the loop-hole as far as possible. With all brushes, when the holes have been properly filled, the ends of the fibres outside are cut with shears, either to an even length or such form as may be desired. The backs are then covered with veneer or other material to conceal the wire and other crudities of the work. In trepanned brushes the bristles are inserted in holes that do not pass right through the stock, and are secured by threads or wires running in drawholes which are drilled through the stock at right angles to them. The ends of these drawholes are plugged so as to be as inconspicuous as possible, and the method avoids the necessity of a veneer on the back. The Woodbury machine, one of the earliest mechanical devices for the manufacture of brushes, which was invented in America about 1870, produced brushes of this kind.

One of the most important purposes to which brushes have been applied is that of sweeping chimneys, and so far back as 1789 John Elin patented an arrangement of brushes for this purpose. Revolving brushes for sweeping rooms were patented in 1811, and the first patent in which they were applied to hair-dressing appears in 1862. Many inventions for sweeping and cleaning roads by means of revolving brushes and other contrivances have been introduced, one of the first being that of Edmund Henning in 1699 for "a new engine for sweeping the streets of London, or any city or town."

Brushes with tufts formed of steel wire are used for cleaning tubes and flues of steam boilers, for the purpose of removing the scale formed by the products of combustion. Steel-wire brushes are also used for cleaning scale from the interior surfaces of a boiler, and for removing the sand from the surface of a casting. Occasionally such brushes are revolved in a machine, for more convenient use on the article to be cleaned or polished. Snyer's patent elastic clutch or coupling, used for such purposes as coupling up or disconnecting a steam-engine from a line of shafting or dynamo, consists essentially of two disks, the adjacent faces of which are provided, one with a ring of brushes made of flat steel wire, the other with a number of finely serrated teeth. One of the disks is movable longitudinally on its shaft, and with the brushes clear of the serrations the clutch is free. On bringing the disks together, which may be done with the engine running at speed, the elasticity of the brush permits the motion to be imparted gradually and without shock to the standing part, until both rotate and are locked together.

These clutches are very powerful, and are capable of transmitting as much as 3000 horse-power.

In dynamo-electric machinery the device used to conduct current into or out of the rotating armature is termed a "brush." There are usually two brushes to each dynamo or motor, and they are placed diametrically opposite, lightly touching the commutator of the armature. It is important that there should be good metallic contact between the brushes and the commutator, and at the same time the frictional resistance resulting from the contact must be a minimum. To effect this result brushes are variously made. A kind of brush frequently used consists of a number of copper wires laid side by side and soldered together at one end, where the brush is held. Brushes are also made of strips of spongy copper cut like a comb, which give a number of bearing points on the commutator. Very good results are obtained from brushes made of copper gauze wound closely until it takes the exterior form of a rectangular block, which is held radially in a spring holder, and bears at the end on the commutator. In place of the gauze block "brushes" of hard carbon blocks are frequently used (see Dynamo).