The amount of magnetic leakage that takes place in the various forms of field-magnet differs greatly in different forms. No doubt there is least waste field in those machines which have the most compact magnetic circuits, fewest joints, and fewest protruding edges and corners. The magnetic lines of the waste field sometimes takes curious forms, which have been experimentally explored, in various types of machines, by Mr. Carl Hering. (Sec Hering's Principles of Dynamo-Eleclric _Machines.) It was stated above that, theoretically, the best cross-section for field-magnet cores was circular, as this gave the greatest area for least periphery, and therefore presumably would for a given lough of wire in the coil give the largest amount of iron to be magnetized. This, of course, means that if the length of wire and the number of turns be given, a core of this section will, of all possible shapes of core, take the greatest number of amperes to bring it to the diacritical point of semi-saturation. Prof. S. P. Thompson discovered, in 1884, that either the electromotive force or the current of every dynamo is proportional to that number of ampere-turns which will bring its core to this diacritical point. This discovery, according to
Thompson. renders it more than ever needful in designing dynamos to adhere as closely as possible to the rule to make the core of circular section whenever the construction will admit of it. Again, as was imbue(' out by Hopkinson, it is a mistake to construct a field-magnet with two or more parallel cores uniting at a common pole-piece: for not only is the wire be tween the two cores useless, it is worse, because it offers ua•stefnl resistance. To divide the iron that might be in one solid cylindrical core into two parallel cylindrical cores implies, of course. that for every turn of wire two turns must be used, each of which is more than half as long as the original one, the total length being increased as a:1, while the magnetizing power is actually reduced. The following calculations of Thompson are added, which show the area (in square centimetres) inclosed in a number of different forms of section, the total periphery of each one being one metre: