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12 RICHARDSON'S BLUEBOOK OF PROJECTION
of current flow throughout the entire circuit has been reversed. This reversal will continue with every halfrevolution of coil AB. (28) This description assumes the flow to be from positive to negative (+ to — ) in accordance with what was until quite recently considered as the action of current. Some scientists now tell us the action is from negative to positive ; in that case the story told in Figure 1 calls only for a reversal of direction of the arows.
(29) In the infinitesimal fraction of a second during which coil AB stands in upright position neither side is cutting any lines of force at all; hence the whole circuit is electrically "dead." The lamps continue to maintain partial illumination through the "dead" period because their incandescent parts cannot wholly cool off in so short a time.
Between magnetic poles N and S, current flows continuously in one direction. It changes direction in the coil and circuit only because with each half revolution of the coil its sides change position, or are reversed. (30) This action is different in multipolar generators, because it takes place every time a coil passes through the field of one of the magnetic poles of the machine. The general effect is the same.
How Current is Commutated
(31) We have tried to give you basic understanding of how current is generated in a dynamo armature, and why an armature always produces alternating current. Since a. c. cannot always be used, means have been provided for changing the armature a. c. into direct current before it reaches the circuit wires. This is called "commutation." It is a means by which a. c. is received at the "commutator" of a generator and there transformed or changed into d. c. It is difficult procedure to describe understandably, but we will do our best to make the matter at least reasonably clear.
In Fig. 2 we have two separate diagrams, A and B. In A we see armature coil AB connected to a commutator ring divided into two sections, EF, upon which brushes