Ans. Across its horizontal diameter into an upper and lower half, as shown in fig. 206, the lower half being seated on, or more frequently cast in one piece with the bed plate. =Ques. What is the objection to dividing a yoke?= Ans. The joints introduced, even if carefully faced and well bolted together, add a little reluctance to the magnetic circuit. [Illustration: FIGS. 213 to 215.--Some methods of attaching detachable cores. The core seat is machined to receive the core, it being necessary to secure good contact in order to avoid a large increase in the reluctance of the magnetic circuit.] =Ques. How does this affect the poles adjacent to the points, and what provision is made?= Ans. It weakens them, and in order to overcome this, the coils of these poles are given a few extra turns. =Ques. How is the reluctance of a yoke joint reduced?= Ans. By enlarging the area of contact; the flange for the bolts furnishes the necessary increase. =Ques. What determines chiefly the cost of field magnets?= Ans. The material used in making the cores and their shape. =Ques. How does this affect the cost?= Ans. Since considerable cross sectional area of core is required, the problem confronting the designer is to design the core by judicious selection of material and shape, that the required number of turns in the magnetizing coil is obtained with the shortest length of wire. [Illustration: FIGS. 216 to 221.--Comparison of field magnet core sections. The shorter the perimeter or outside boundary of the core for a given cross sectional area, the less will be the amount of copper required for the magnetizing coils. All the above sections are of equal area, and the figures marked on each represent relative values for the perimeters, the circle for convenience being taken at 100.] =Ques. What is the principal objection to the use of cast iron for core construction?= Ans. Since its sectional area must be considerably more than wrought iron, a much greater quantity of copper is required for the magnetizing coils. Copper is expensive, while cast iron cores are less expensive than equivalent ones of wrought iron; in this connection, it is interesting to observe how different designers aim at true economy in construction. Steel is sometimes used in place of wrought iron, and though less efficient magnetically, it can be cast into the desired shape, thus avoiding the somewhat expensive processes of forging and machining, which are necessary in the case of wrought iron. =Ques. What form of core requires the least amount of copper for the magnetizing coils, and why?= Ans. The cylindrical core, because it has the shortest periphery or boundary for a given area enclosed.