During the second half of the revolution, figs. 171 to 173, the changes that occur are the same as in the first half, with the exception that the current is in the reverse direction; these changes are as shown by the curve from G to I.
CHAPTER XIV
THE DYNAMO: CURRENT COMMUTATION
How the Dynamo Produces Direct Current: The Commutator.—The essential difference between an alternator and a dynamo is that the alternator delivers alternating current to the external circuit while the dynamo delivers direct current. In both machines, as before stated, alternating currents are induced in the armature, but the kind of current delivered to the external circuit depends on the manner in which the armature currents are collected.
In the case of an alternator, the method is quite simple. As previously explained, each end of the loop is connected with an insulated collector ring carried by the shaft, the current being collected by means of brushes which bear against the rings. This principle, rather than the actual construction, is shown in the preceding illustrations. Its important point, as distinguished from other methods of collecting the current, is that each end of the loop is always in connection with the same brush.
Ques. How is direct current obtained in a dynamo?
Ans. A form of switch called the commutator is placed between the armature and the external circuit and so arranged that it will reverse the connections with the external circuit at the instant of each reversal of current in the armature.
Ques. How is a commutator constructed?
Ans. It consists of a series of copper bars or segments arranged side by side forming a cylinder, and insulated from each other by sheets of mica or other insulating material.