There may be four, six, eight, or more poles, arranged in alternate order around the armature. Fig. 204 shows a four pole field magnet having a common yoke or iron ring, with four pole pieces projecting inwardly, and over which the exciting coils are slipped. In the larger machines the yoke is made in two parts bolted together as shown in fig. 206, so that the upper portion may be lifted off for examination of the armature. =Ques. Can the number of poles in a multi-polar machine be advantageously increased to 16, 32, or more?= Ans. A large number of poles is not advisable except in very large machines, since it involves an increase in the expense of machine work, fittings, etc., somewhat out of proportion to the reduction in cost of material and increase in efficiency. =Ques. What materials are generally used for field magnets?= Ans. Wrought iron, steel and copper. There are a number of considerations which govern the selection of the materials to be used in a particular machine, such as initial cost, weight, efficiency, etc. [Illustration: FIGS. 205 and 206.--Solid and split construction of yoke for multi-polar dynamos. In the latter type the yoke is in two halves joined along a horizontal diameter; while the upper half may be conveniently removed to give access to the armature, it has the disadvantage of the joint, which, no matter how well made, will add to the reluctance of the magnetic circuit. The figures also illustrate the circular and segmental forms of yoke construction.] =Ques. In the construction of field magnets, what governs the choice of materials?= Ans. For cores, wrought iron is most desirable, as requiring the smallest amount of material for a given flux. There is a saving in copper due to using wrought iron for the core since, on account of its small size, the length of each turn of the magnetizing coil is reduced. For heavy yokes, where lightness is not essential, but very often the reverse, cast iron is used, as its cross section can be made larger than that of the cores, this increase in area serving to give strength and rigidity to the machine. Cast steel occupies a place intermediate between cast iron and wrought iron both in cost and magnetic properties. [Illustration: FIGS. 207 to 209.--Various sections of cast iron yoke. In form, these yokes may be either circular or segmental as shown in figs. 205 and 206.] [Illustration: FIGS. 210 to 212.--Various sections of cast steel yoke. The ribs shown in figs. 210 and 211 are provided to secure stiffness.] =Ques. Name two forms of yoke in general use.= Ans. The solid, and divided types as shown in figs. 205 and 206. =Ques. What is the object of dividing a yoke?= Ans. To permit access to the armature, where the construction does not admit of removal of the latter from the side. =Ques. How is the yoke usually divided?=