Com′mutator. In most kinds of electrical work it is necessary at times to change the direction of the current in some part of the circuit. The commutator is an instrument for producing this change. The device represented in the accompanying figure is typical of all commutators. The block A, B, C, D is made of wood, and has four holes bored about half-way through it. These holes, placed one at each corner of a square, act as cups to hold mercury.
Into the sides of the block are inserted four wires, E, F, G, H. Each of these wires connects with the mercury in the cup nearest it. The poles of the battery are joined to two diametrically opposite cups, say B and D; the wires from the rest of the circuit, say from a galvanometer, are joined to the remaining two cups, A and C. For closing the circuit two short, thick, copper conductors P and Q, are mounted on a movable block, as shown in the figure. These connectors, P and Q, may be placed in the mercury cups in such a way as to connect B with A, and hence C with D; or they may be placed so as to connect B with C, and hence A with D. In the one case the current flows from G to E through the galvanometer; in the other case from E to G. To change the direction of the current in the galvanometer circuit, we have, therefore, only to lift the top block and rotate it through 90° in either direction. Such an instrument is called a commutator. In many kinds of electrical machinery the commutator is made to work automatically. Thus, in the case of the dynamo, the currents which are generated in the coils of the armature are alternating, but a commutator is in many instances placed on the shaft of the armature, which automatically reverses the connections of the armature-coil and the external circuit, so that the current in the external circuit always flows in one direction. Such a dynamo is called a direct-current generator. The dynamo which is not provided with a commutator is called an alternate-current generator.