We have already noticed that in the case of a single coil the current rises and falls in a series of pulsations. Such a form of armature would be unsuitable for large dynamos, which accordingly have a number of coils wound over their drums, at equal distances round the circumference, and a commutator divided into an equal number of segments. The subject of drum winding is too complicated for brief treatment, and we must therefore be content with noticing that the coils are so connected to their respective commutator segments and to one another that they mutually assist one another. A glance at Fig. 73 will help to explain this. Here we have in section a number of conductors on the right of the drum (marked with a cross to show that current is moving, as it were, into the page), connected with conductors on the left (marked with a dot to signify current coming out of the page). If the "crossed" and "dotted" conductors were respectively the "up" and "down" turns of a single coil terminating in a simple split commutator (Fig. 69), when the coil had been revolved through an angle of 90 deg. some of the up turns would be ascending and some descending, so that conflicting currents would arise. Yet we want to utilize the whole surface of the drum; and by winding a number of coils in the manner hinted at, each coil, as it passes the zero point, top or bottom, at once generates a current in the desired direction and reinforces that in all the other turns of its own and of other coils on the same side of a line drawn vertically through the centre. There is thus practically no fluctuation in the pressure of the current generated. The action of single and multiple coil windings may be compared to that of single and multiple pumps. Water is ejected by a single pump in gulps; whereas the flow from a pipe fed by several pumps arranged to deliver consecutively is much more constant. MULTIPOLAR DYNAMOS. Hitherto we have considered the magnetic field produced by one bi-polar magnet only. Large dynamos have four, six, eight, or more field magnets set inside a casing, from which their cores project towards the armature so as almost to touch it (Fig. 74). The magnet coils are wound to give N. and S. poles alternately at their armature ends round the field; and the lines of force from each N. pole stream each way to the two adjacent S. poles across the path of the armature coils. In dynamos of this kind several pairs of collecting brushes pick current off the commutator at equidistant points on its circumference. [Illustration: FIG. 74.--A Holmes continuous current dynamo: A, armature; C, commutator; M, field magnets.] EXCITING THE FIELD MAGNETS.