[Illustration: FIGS. 222 to 225.--Several forms of pole piece. Where the extremities project as in figs. 222 and 223, they are called _horns_. The object of these is to reduce the reluctance of the air gap. The width of “fringe” of the magnetic field is influenced by the shape of the pole piece; the margin of fringe should be such that the flux density will vary from zero to a high value where the inductors enter.] Figs. 216 to 221, show a series of cross sections, all of the same area. The number marked on each section indicates the length of the boundary line, that of the circle being taken for convenience as 100. =Ques. What are the pole pieces?= Ans. These are the end portions of the field magnets, joined to, or cast together with the core and placed adjacent to the armature. The faces of the pole pieces are of circular shape, thus forming the sides of the so-called armature chamber within which the armature rotates. [Illustration: FIG. 226.--Unsymmetrical pole piece introduced by Gravier to concentrate the magnetic field. When the dynamo is working at small loads, the flux in the gap is nearly uniform, but at heavy loads, the distortion due to the armature current forces the flux forward and saturates the forward horn, thus preventing much change in its flux density, on account of the saturation, and the diminishing area. Lundell combined the unsymmetrical and slotted forms of pole piece as shown in fig. 237.] =Ques. Why are the pole faces made larger than the coils?= Ans. In order to reduce the reluctance of the air gap between the face and the armature, thus enabling fewer magnetizing coils to be used. [Illustration: FIG. 227.--Pole piece with oblique slots; a modification of Lundell’s form of pole piece as suggested by Thompson. In operation, the neck of the casting becomes saturated and offers considerable reluctance, which tends to prevent distortion of the magnetic field.] It is important that the field should be magnetically rigid, that is, not easily distorted. This stiffness of field can be partially secured by judicious shaping of the pole pieces. A few forms of pole piece are shown in figs. 222 to 231. If the projecting tips of the pole pieces, or _horns_ as they are called, be widely separated, as in fig. 222, they are not always good, even though thin. It is better that they should be extended as in fig. 223 so that they may be saturated by the leakage field or else cut off as in fig. 224. An extreme design, suggested by Dobrowolski, as shown in fig. 225, surrounds the armature with iron. [Illustration: FIG. 228.--Non-concentric pole faces; one method of securing suitable magnetic “fringe” with fair magnetic rigidity of field.] [Illustration: FIGS. 229 to 231.--Various shapes of pole piece for securing a gradual entrance of the armature inductors into the magnetic field.]