=304. The Commutator.=--For a dynamo to deliver a _direct current_ it must carry upon the shaft of the armature a _commutator_. The commutator is used to _reverse_ the connections of the ends of the armature coils at the instant that the current changes its direction in the armature. This reversal of connection when the direction of current changes, keeps the current in the outside circuit flowing in the same direction. Fig. 291 is a diagram of an armature with a commutator. The commutator is a _split ring_, having as many parts or _segments_ as there are coils upon the armature. The brushes touch opposite points upon the commutator as they slide over the surface of the latter. Suppose that the armature viewed from the commutator end rotates in a counter-clockwise direction, also that the currents from the upper part move toward the commutator and out the top brush. [Illustration: FIG. 291.--The armature coils are connected to a commutator producing a direct current.] As the armature revolves, its coils soon begin to cut the force lines in the opposite direction. This change in the direction of cutting the lines of force causes the current to reverse in the coils of the armature. At the instant the current changes in direction, what was the upper segment of the commutator slips over into contact with the lower brush, and the other segment swings over to touch the upper brush. Since the current has reversed in the coils it continues to flow out of the upper brush. This change in connection at the brushes takes place at each half turn of the armature, just as the current changes in direction in the coils. This is the manner in which the commutator of a dynamo changes the alternating current produced in the armature coils, into a direct current in the external circuit. Fig. 292 (1) represents graphically an alternating current, (2) of the same figure shows current taken from the brushes of the commutator of a dynamo with one coil on the armature. [Illustration: FIG. 292.--Graphic representation of (1) an alternating current; (2) a pulsating current; (3) a continuous current.] [Illustration: FIG. 293.--DeLaval multi-stage turbine and gear driving 750-kw., 750-r.p.m., 600-volt direct-current generator.] A practical dynamo, however, has many coils upon its armature with a corresponding number of segments upon the commutator. (See Figs. 289 and 293.) As each coil and commutator segment passes a brush, it contributes an impulse to the current with the result that armatures with many coils produce currents that flow quite evenly. (See Fig. 292, 3.) The current represented in Fig. 292 (2) is called a _pulsating_ current. [Illustration: FIG. 294.--A wire carrying a current across a magnetic field is pushed sideways by the field.]