In the classification of dynamos, those of the first class required a single approximately uniform field of force, whilst those of the second required a complex field of force differing in intensity and sign at different parts. Hence a corresponding general demarcation between the field - magnets in the 2 classes of machine. In the first, are usually 2 pole - pieces on opposite sides of a rotating armature; in the second, a couple of series of poles set alternately round a circumference or crown, the coils which rotate being set upon a frame between 2 such crowns of poles.
Confining attention to the first class of machines, in practice their magnets differ widely in construction and design.
In very few of the existing patterns is much trouble taken to secure steady magnets, by making them long, heavy, and solid, or with very heavy pole - pieces. Repeatedly an unnecessary amount of wire has been wound upon field - magnets; and the usual excuse is that, with less wire, the machine does not work so well. If, however, it is found necessary to wind on so many coils upon the magnets as to bring these practically to saturation long before the machine is doing its maximum work, it is clear that either the iron is insufficient in quantity or it is deficient in quality. In the Burgin machines, where cast - iron field - magnets are employed, the smaller magnetic susceptibility of this metal is made up for by employing a great weight of it. In Siemens'8 smaller dynamos, the amount of iron employed in the field - magnets would be quite insufficient if it were not of high quality; as it is, Prof. Thompson is of opinion that the mass of it (especially in the polar parts) might with advantage be increased. In some of the early machines of Wilde, and in Edison's well - known dynamos, long field - magnets, with heavy pole - pieces, are found.
Edison's dynamos, indeed, are all remarkable in this feature; the pole - pieces and the yoke connecting the iron cores of the coils are made abnormally heavy. This is not more noticeable in the giant dynamos used at the Holborn Viaduct, than in the smaller machines used in isolated installations for 60 and for 15 lights.
The principle of shaping the magnets, so that their external form approximates to that of the magnetic curves of the lines of force, is to some extent carried out in such widely differing types of machine as the Gramme with " Ja - min " magnet, the Jurgensen dynamo, and Thomson's " mousemill " dynamo. The 2 machines last named exhibit several curious contrasts. In the Jurgensen, the field - magnets have heavy pole - pieces; in the Thomson, are none; and in the Thomson, the iron core is thicker at the middle than at the ends. In both, are auxiliary internal electromagnets, fixed within the rotating armature, to concentrate and augment the intensity of the field, according to the device patented by Elphinstone and Vincent. In the Thomson machine, the coils are heaped on more thickly at the middle of the field - magnets; in the Jurgensen, the coils are crowded up around the poles. Judging from a report on this machine by Professors Ayr - ton and Perry, the arrangement is not satisfactory in practice, as there are more coils than suffice to magnetize the magnets.
Another suggestion, indicated from theoretical considerations, was that of laminating the pole - pieces, to prevent the production in them of wasteful Fou - cault currents. But one machine has been designed in which this precaution is carried into effect. This is the disk - dynamo of Drs. Hopkinson and Muir - head, the field - magnets of which are made up of laminae of iron, cast into a solid iron backing.
Another matter is the form to be given to pole - pieces, in order to produce the best effect. These present such singular divergence in practice as to suggest the thought that little importance has been attached to them. Yet upon the form and extent given to the pole - pieces depend the reduction of idle wire in the armature, the reduction of sparking at the commutator, and the avoidance of counter - electromotive forces in the armature. If the pole - pieces are badly shaped for their work, or approach one another too far round the armature, they may completely perturb the approximate uniformity of the field, and may cause the central portion of the field to be of much weaker intensity than the two lateral regions between the edges of the pole - pieces. When this is the case, the rotating coils are virtually moving in a double field, and it is even possible that, in consequence, the direction of the currents induced in the individual coils may be reversed 4 or 6 times as they make one rotation.
In such a case, the distribution of potential round the separate bars of the commutator will be abnormal.
The armatures of dynamos of the first class may be roughly classified in 3 groups, according to the manner of arranging the coils; these are - -
(1) Ring armatures, in which the coils are grouped upon a ring, whose principal axis of symmetry is its axis of rotation also.
(2) Drum armatures, in which the coils are wound longitudinally over the surface of a drum or cylinder.
(3) Pole armatures, having coils wound on separate poles, projecting radially all round the periphery of a disk or central hub.
To these will be added a fourth form, disk armatures, when dealing with dynamos of the second class.
The object of all these combinations is to obtain practical continuity of current. Some of the individual coils should be moving through the position of maximum action, whilst others are passing the neutral point, and are temporarily idle. Hence a symmetrical arrangement around an axis is needed. Ring armatures are adopted in practice in the dynamos of Paciuotti, Gramme, Schuckert, Gulcher, Fein, Heinrichs, De Meritens, Brush, Jurgensen, and others. Drum armatures are found in the Siemens (Alteneck), Edison, Elphinstone - Vincent, Laing, and other machines. Pole - armatures are used in the dynamos of Allen, Elmore, and of Lontin. There are several intermediate forms. The Burgin armature consists of 8 or 10 rings, side by side, so as to form a drum. The Lontin (continuous - current dynamo) has the radial poles affixed upon the surface of a cylinder. The Maxim armature is a hollow drum wound like a Gramme ring, and has therefore a great quantity - of idle wire on the inner surface of the drum.