It may be necessary to reverse the direction of rotation of a dynamo, if the driving pulley to which it has to be connected happen to revolve left handed, or if it be necessary to bring the loose side of the belt on top of the pulley, or to place the machine in a certain position on account of limited space. The direction of rotation of ordinary series, shunt, or compound bipolar dynamos may be reversed by simply reversing the brushes without changing any of the connections, then changing the point of contact of the brush tips 180°. In multipolar dynamos, a similar change, amounting to 90° for a four pole machine, and 45° for an eight pole machine, will reverse their direction of rotation. It will be understood that under these conditions, the original direction of the current and the polarity of the field magnets will remain unchanged. This rule does not apply to arc dynamos and other machines, which have to be run in a certain direction only, in order to suit their regulating devices. If the direction of current generated by a dynamo be opposite to that desired, the two leads should be reversed in the terminals, or the residual magnetism should be reversed by a current from an outside source. [Illustration: Fig. 670.--Method of assembling core discs. For this operation two wooden "horses" should be provided to support the core at a convenient height, as shown in the illustration.] Starting a Dynamo.--Having followed the foregoing instructions, all keys, spanners, bolts, etc., should be removed from the immediate neighborhood of the machine, and the dynamo started.