The field magnet, in addition to furnishing the magnetic field, has to do duty as a framework which often involves considerations other than those respecting maximum economy. =The Make Up of a Field Magnet.=--In construction, the electromagnet, used for creating a field in which the armature of a dynamo revolves, consists of four parts: 1. Yoke; 2. Cores; 3. Pole pieces; 4. Coils. These are shown assembled in figs. 201 to 204. =Ques. What is the object of the yoke?= Ans. The yoke serves to connect the two “limbs,” that is, the cores and pole pieces, and thus provide a continuous metallic circuit up to the faces of the pole pieces. =Ques. How is the yoke constructed?= Ans. It usually forms the frame of the dynamo as shown in figs. 205 and 206. [Illustration: FIG. 201.--Salient pole, bipolar field magnet with single coil wound around the yoke.] =Ques. What may be said of the cores?= Ans. The cores, which are usually of circular form, carry the coils of insulated wire used to excite the magnets. =Classes of Field Magnet.=--Although numerous forms of field magnet have been devised, they can be classed into two groups according to the type of pole, as: 1. Salient pole; 2. Consequent pole. The distinction between these two types of pole is shown in figs. 201 to 203. By inspection of the figures, it will be seen that the term _salient_ applies to poles produced when the pole pieces form the _ends_ of the magnet, as distinguished from _consequent_ poles, or those formed by coils wound on a continuous metal ring or equivalent. In the salient pole bipolar magnet, the winding may be either upon the limbs, M M fig. 202, or upon the yoke, Y as shown in fig. 201. The magnetic circuit of salient and consequent poles is indicated in the figures by the dotted lines. [Illustration: FIG. 202.--Salient pole, bipolar field magnet with two coils wound around the cores.] [Illustration: FIG. 203.--Consequent pole, bipolar field magnet with two coils on the cores. This is known as the “Manchester” type in which the cores are connected at the ends by two yokes--so named from its original place of manufacture at Manchester, England.] =Multi-Polar Field Magnets.=--In the multi-polar machine, the subdivision of the magnetic flux reduces the amount of material of both magnet and armature. Moreover, there is less heating on account of the greater capability of dissipating the heat, offered by the increased area of surface per unit of volume in each magnet pole and winding. [Illustration: FIG. 204.--Modern dynamo with four consequent pole field magnets. In this construction the ring shaped yoke also serves as a frame; the circular form of yoke gives the least chance for magnetic leakage.]