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.
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.