ARMATURE WINUINGS.—In the ring, disk, and pole type of a-rmature the winding or wind ings are practically continuous and symmetrical. but in the drum-armatnre there is touch scope for devising combinations, in order to secure, first, an equal length of wire in each coil ; and, second, the shortest length of wire capable of giving the required E. M. F. Figs. 3, 4, 5, 6, and 7 show various methods of artnature-winding employed by different constructors. The early armatures of Ilefner-Alteneck (Fig. 4) were wound unsymmetrically, on account of the simple construction, and better insulation of which it permitted. Subsequently Froelich in vented a symmetrical winding (Fig. Iireguet has designed a large number of windings, among them, Figs. 5 and 6, mid showed that with eight commutator segments eight different symmetrical windings were possible; that winding should, of course, be selected which, with the same inductive capacity, will hare the shortest length of wire. Ilreguet calculates that for the ends of the Bruins the following lengths of wire are necessary for the various systems: Froelich winding. Hefner-Alteneck winding, Breguet winding, 26; Ilregnet (an other) winding, 28'4; Fig. 7 shows one style of winding of the Edison armature which is symmetrical, but which has an uneven number of divisions. Fig. 8 shows a diagram of one type of the Weston winding, and Fig. 9 a section through the armature : it will be noted that the layers of wire successively change from outer to inner, thus equalizing the potential gen erated in each.
There are also various methods of winding closed coil ring armatures. The simplest, of course, is to wind as many sections as there are collector-rings. and connect the junction of con tiguous coils to each har. Another method is to wind twice as many sections :ts t here are bars in the collector. each section being united either in series or parallel with that diametrically oppo site it, and the pair so united being treated as a single section in the coupling up of the ring. When ring-armat nres are employed in in ul tipt d r fields, a variety of methods of connection are possible. That of Mordey is shown in Fig. 10. It consists in adding to the usual Gramme winding a system of cross-connections between those portions of the armature-cirenit which arrive simulta neously equal potentials. This may he done by cross-connecting either the hags of the collector or the wires of the winding. In O pole machines each bar must communicate with that situated 180° from it; in 6-pole machines. with those situated at 120' from it. Moriley's method, as applied to at 4-pole machine, is shown in Fig. 10, which connections of a simple ring. It will be
noted only two brushes, and these at 90° apart, are reqniftd to col leet. the currents.
Another method sigi.c.est od by Prof. Perry, in 1882. is applieable only to armatnres wound with nu odd number of sections. The dia gram in Fig. 11 relates to an 11-part armature in a 4-pole machine. In this method the suc cessive sections of the coil are not connected together, as in Gramme's winding, but each coil is connected across to that coil which lies nearest the diametrically opposite point, or, if there are c sections, each section is connected to the sections -1) beyond. The coils still form a closed circuit, but the total electromotive force from brush to brush is the sum of the electromotive forces in half the coils, while in Mordey's method it is but one quarter. Alordey's method has the contrary advantage of reducing the resistance to one quarter, and is preferable for low potential machines.
FIELD 3IAGNETS.—While the employment of permanent mag nets, as in the older types of machines, would involve no expendi ture of energy in order to obtain the required magnetic field, they are now practically discarded, as the field which can be produced by them is weak, compared with that which can be produced by eleetro-magnets. In all modern machines, therefore, the latter are employed. In the construction of magnets for dynamos, iron alone can be employed, on account of its high magnetic suscep tibility. Present practice is now tending largely to the employ ment of wrought iron, on account of the greater magnetic power developed in it for a given current. The softer the iron the greater its susceptibility, the ratio of cast iron to wrought iron being, approximately, as 2 to 3 ; that is, a given magnetizing force will create 50 per cent more lines of force in wrought than in cast iron. In designing field-magnets care should be taken to make them of ample size, so that they may not become too quickly satura ted ; and it is desirable to have the magnet as thick as possible, so that it may react slowly to changes in the main circuit, and thus steady the current induced in its 'field. Magnets should. theoretically, be so constructed that they may receive the highest magnetizing effect with the shortest possible winding; this is the case when the magnet is circular in shape. Sonic constructors, however, employ rectangular slabs of wrought iron, as they are cheaper than circular cores of equal cross-section. In the design of field-magnets, care should be taken to avoid all sharp corners, as the magnetism strays or escapes from such points into tl:e air and is wasted. The laws of distribution of magnetism follow closely those of static elec tricity.