CHAP. Ill] HYSTERESIS AND EDDY CURRENTS 41 



induced along closed paths of least resistance linked with the flux. 

 These currents permeate the whole bulk of the iron and are called 

 eddy or Foucault currents. Eddy currents cause a loss of energy 

 which must be supplied either electrically or mechanically from 

 an outside source. Therefore, the iron cores used for variable fluxes 

 are usually built of laminations, so as to limit the eddy currents 

 to a small amount by interposing in their paths the insulation 

 between the laminations. Japan, varnish, tissue paper, etc., are 

 used for this purpose. In many cases the layer of oxide formed 

 on laminations during the process of annealing is considered to be 

 a sufficient insulation against eddy currents. 



The usual thickness of lamination varies from 0.7 to 0.3 mm., 

 according to the frequency for which an apparatus is designed, 

 the flux density to be used, the provision for cooling, etc. The 

 more a core is subdivided the lower is the loss due to eddy currents, 

 but the more expensive is the core on account of the higher cost 

 of rolling sheets, and of punching and assembling the laminations. 

 ies, more space is taken by insulation with thinner stampings, 

 so that the per cent net cross-section of iron is reduced. The 

 net cross-section of laminations is usually from 95 to 85 per cent 

 of the gross cross-section, depending upon the thickness of the 

 laminations, the kind of insulation used, and the care and pres- 

 sure used in assembling the core. For preliminary calculations 

 about ten per cent of the gross cross-section is assumed to be 

 lost in insulation. 



Fig. 9 shows two iron cores in cross-section, one core solid, 

 the other subdivided into three laminations by planes parallel 

 to the direction of the lines of force. The lines of force are shown 

 by dots, and the paths of the eddy currents by continuous lines. 

 Eddy currents are linked with the lines of force, the same as the 

 current in the exciting win.lin-j. In fact, eddy currents are similar 

 t<> the secondary mm-nts in a trannt inasmuch as they 



tend to reduce the flux created by the primary current. The 

 core must be laminated in planes perpendicular to the lines of 

 flow of the eddy currents, so as to break up their paths and at the 

 same time not to interpose air-gaps in the paths of the lines of 

 farce, 



An iron core can be further subdivided by using thin imn 

 wires in place of laminations. Such cores were used in early 

 machines and transformers, but were abandoned on account of 



