1. _Self-exciting machines_ of which the magneto is the simplest. Its magnetic field is obtained from permanent magnets, hence the electromotive force generated is comparatively small. The more important type of self-exciting machine is provided with electromagnets in which the field of force is “built up” from the residual magnetism of the soft iron or steel cores of the field magnets of the dynamo itself. Nearly all commercial types of dynamo are of this class. 2. _Separately excited machines_ in which the field magnets are magnetized when the machine is in operation by current supplied from a separate source such as a battery or magneto generator. With respect to the third division, based on the field winding, dynamos are classed as: 1. Series wound; 2. Shunt wound; 3. Compound wound. In addition to the foregoing there are further distinctions with respect to the mechanical features. Most dynamos have a revolving armature and stationary field magnets; however, in some cases, both the armature and field magnets are stationary, a revolving iron inductor being provided to intercept the magnetic lines intermittently which produces the same effect as is obtained in cutting the magnetic lines by a revolving armature. =Ques. What may be said of bipolar and multi-polar dynamos?= Ans. Dynamos with bipolar field magnets were universally used prior to 1890, but since that time machines of this type are only made in very small sizes; the multi-polar dynamo is the type now in general use. =Ques. State some of the features of the multi-polar dynamo.= Ans. In this class of machine, the armature and field magnets are surrounded by a circular frame, or _ring yoke_ to which the field magnets are attached. This ring arrangement has the advantages of strength, simplicity, symmetrical appearance, and minimum magnetic leakage, since the pole pieces have the least possible surface and the path of the magnetic flux is shorter. =Ques. What important advantage is gained by the use of multi-pole field magnets?= Ans. Commercial voltages are obtained at moderate armature speed. The difficulty experienced with bipolar machines is that, with a dynamo of large output, the speed at which its armature would have to rotate to generate commercial voltages would be excessive. [Illustration: FIGS. 188 and 189.--Circuit diagrams to illustrate the difference between a dynamo and a magneto. The former has its field magnets F F magnetized by means of a small current flowing around a shunt circuit. In a magneto the field magnets are permanently magnetized. The strength of the magnet field of a magneto is constant while that of a dynamo varies with the output.]