[Illustration: FIG. 234.--Fort Wayne laminated pole piece before being cast welded into frame. In the faces of solid pole pieces there exist minute electric currents called _eddy currents_ which cause heating of the iron and increase the energy required to maintain a magnetic circuit in much the same manner as does reluctance. This loss is reduced by dividing the magnetic circuit in the line of flux into numerous parallel paths separated by some material of relatively high resistance. In construction, the above core and pole piece is made up of sheets of annealed steel of two different widths assembled together to form proper size and shape. The minute spacing between these laminations and the slight oxidizing on each surface is sufficient to reduce considerably the eddy currents. By cast welding the pole piece into the frame, a low reluctance is secured.] =Ques. What is a laminated pole?= Ans. One built up of layers of iron sheets, stamped from sheet metal and insulated, as shown in fig. 234. =Ques. What may be said of this construction?= Ans. It is a most approved method, and one frequently employed in the construction of cores and pole pieces. Fig. 234 shows a combined core and pole piece made entirely of sheet iron punchings assembled and riveted together, and fig. 235, a core to be used with separate pole piece. It should be noted that in both cases there is a longitudinal slot extending from the end into the core. This was first suggested by Lundell, the object being to prevent, as far as possible, the distortion of the magnetic field due to armature reaction especially on heavy overloads. [Illustration: FIG. 235.--Fort Wayne laminated core without pole piece, as used on large dynamos. It is constructed of punchings from sheet iron, and riveted under pressure. The alternate end projections and grooved base insure good mechanical union of metal in cast welding to magnet frame. Reluctance between core and yoke is reduced to a minimum by cast welding. The core is slotted parallel with the shaft to prevent, as far as possible, the distortion of the magnetic field, especially on heavy overloads.] =Ques. What mode of construction is adopted to reduce the reluctance of the magnetic circuit when laminated poles are used?= Ans. They are cast welded into the frame. [Illustration: FIG. 236.--Fort Wayne one piece frame with cast welded combined cores and pole pieces. In any electrical apparatus a magnetic circuit of low reluctance requires less energy to maintain a given flux than one having a comparatively high reluctance. To reduce this to a minimum the pole pieces and cores are combined into one part and then cast welded into the yoke or frame. Thus the continuity of the magnetic circuit is practically unbroken save for the air gap.]