As soon as the loop rotates out of the vertical plane, the electromotive force rises and the current begins to flow in the direction indicated by the arrows, going out to the external circuit through brush M, and returning through brush S. Continuing the rotation, the electromotive force increases in proportion to the sine of the angle made by the plane of the loop with the horizontal, until the loop comes into the horizontal position illustrated in fig. 170. This increase is indicated by the gradual rise of the sine curve from E to F. The loop has now made one quarter of a revolution and the electromotive force reached its maximum value. As the loop rotates past the horizontal position of fig. 170, the electromotive force gradually decreases in intensity, reaching the zero point at the end of the second quarter--that is, when the loop has turned one half revolution. This is indicated by the gradual fall of the curve from F to G. When the loop turns out of the vertical position shown in fig. 171 the current reverses, because the movement of A B and C D is reversed; at this instant the brush M becomes negative, and S positive. This reversal of current is indicated by the curve falling _below_ the axis from G to I. 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?=