24 ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 
THE MAGNET SYSTEM. 
47. The Magnet System.—The magnet wheel consists of a. 
_ east iron or cast steel spider, with the pole pieces which are 
of solid steel all through, or with a solid magnet core but 
laminated pole shoes, or of laminated sheet iron all 
through. 
Cast steel poles can be used in connection with closed or 
half open slots, but laminated pole shoes are to be applied 
with an armature of open slots in order to reduce the eddy 
currents and losses in the pole faces. The thickness of 
the lamination should be 0.04 inch and between the sheets 
some good insulating varnish or the oxide insulation of 
the plate itself is sufficient. 
The ratio between the effective iron section and the total 
iron section is about 0.95. 
48. Solid Poles.—Generally speaking the cost of manufacture 
of solid cast poles or laminated ones is about the same. 
The advantage resulting from the application of solid 
pole pieces, consists in the damping effect of the same, so 
improving the parallel working of the alternators; and, 
further, that for high speed machines, the fixing of the 
solid pole pieces is more reliable, as strong bolts may be 
used. 
Another advantage of the solid pole piece, is that the section 
ean be made oval or circular, so redueing the leakage of 
the field system. The amount of field copper is also 
reduced, as the peripheral length is less for a certain cross 
section of oval form than the length for the same eross 
section of rectangular form. By the use of an oval section 
in certain designs the economy in copper is sometimes 
considerable. 
49: Laminated Poles.—With laminated pole pieces, the 
magnet wheel is more certain to run true and to keep its 
