The frame end of the core as shown in the illustrations has irregularities in the heights of the different sheets, as well as grooved undercut surfaces, in order to enable the molten metal of the frame to key well into the laminations of the core, making a good joint, both mechanically and electrically. By this construction, the continuity of the magnetic circuit is practically unbroken save for the air gap between the pole piece and armature. Fig. 236 shows a one piece frame of a six pole dynamo having cast welded into it, combined cores and pole pieces. =Ques. What is the disadvantage of laminating a core?= Ans. It necessitates a nearly square or rectangular section, which requires more copper for the winding than the cylindrical form. [Illustration: FIG. 237.--Lundell type of combined core and pole piece; a combination of Gravier’s unsymmetrical horns and longitudinal slot designed to prevent distortion of field.] =The Magnetizing Coils.=--The object of the magnetizing coils, is to provide, under the various conditions of operation, the number of _ampere turns_ of excitation required to give the proper flux through the armature to produce the desired electromotive force. With respect to the manner in which magnetizing coils are wound they are said to be: 1. Spool wound; 2. Former wound. =Ques. Describe the methods of constructing spool wound coils.= Ans. The spool is made in various ways, sometimes entirely of brass, or of sheet iron with brass flanges, or of very thin cast iron. Some builders use sheet metal with a flange of hardwood, such as teak. If a spool be simply put upon a lathe to be wound, the inner end of the wire, which must be properly secured, should be brought out in such a way that it cannot possibly make a short circuit with any of the wires in the upper layers. To avoid this difficulty, the wire is sometimes wound on the spool in two separate halves, the two inner ends of which are united, so that both the working ends of the coil come to the outside as shown in fig. 238. [Illustration: FIG. 238.--Method of winding magnet spool so that the two ends of the coil will come to the outside. This method has also been used for induction coils, where it is desirable to keep the ends of the wire away from the core and primary coil.] =Ques. Describe the construction of former wound coils.= Ans. Former wound coils are wound upon a block of wood having temporary flanges to hold the wire together during the winding. Such coils have pieces of strong tape inserted between the layers and lapped at intervals over the windings to bind them together. Coils are usually soaked with insulating varnish and stove dried. =Ques. What may be said with respect to the coil ends?=