THE MAGNETIC CIRCUIT [ART. 11 



a secondary coil connected to a calibrated ballistic galvanometer. 

 The exciting ampere-turns divided by the average length of the 

 path give the magnetic intensity H. The total flux divided by the 

 cross-section of the iron path gives the value of the flux density B, 

 which is plotted as an ordinate against H for an absissa. Similar 

 tests arc made for other values of H and B; the results give the 

 magnetization curve of the material. In other words, a magneti- 

 zation curve gives the relation between the magnetomotive force 

 and the flux for a unit cube of the material. By combining unit 

 cubes in series and in parallel a relationship is established 

 between flux and ampere-turns for a circuit of any dimensions, 

 made of the same material. 



The curves shown in Fig. 2 refer to the following materials : -(a) 

 Cast iron, which is used as the magnetic material in the stationary 

 field frames of direct-current machines, and in the revolving-field 

 spiders of low speed alternators. It is evident from the curves 

 that cast iron is magnetically much inferior to steel ; but it is used 

 on account of its lower cost and ease of machining. (6) Cast steel, 

 which is used for pole pieces, plungers of electromagnets, etc. 

 It is used also for the field frames of such machines in which 

 economy of weight or space is desired, for instance, in railway 

 and crane motors, and in machines built for export, (c) Forged 

 steel, which is used for the revolving fields of turbo-alternators, 

 on account of the considerable mechanical stresses developed in 

 such high speed machines by the centrifugal force. 



The curves in Fig. 3 refer to carbon-steel laminations and to 

 silicon-steel laminations. The former is used in the armatures of 

 direct and alternating-current machines, the latter mainly in trans- 

 formers. There is not much difference between the two kinds with 

 regards to their B H curves, but silicon steel shows a much lower 

 loss of energy due to hysteresis and eddy currents (see Art. 20 

 below). A material of much higher permeability is used for 

 armature cores, when it is desired to use very high flux densities 

 in the teeth. A magnetization curve for such steel laminations is 

 shown in Fig. 28. 



For convenience and accuracy the lower part of each curve in 

 Fig. 2 is plotted separately to a larger scale, " A," while the upper 

 parts are plotted to a smaller scale, " B." Thus, Fig. 2 contains 

 only three complete magnetization curves. The curve for silicon- 

 steel laminations in Fig. 3 is also plotted to two different scales, 



