Fig. 11—Complete Magnetic Circuit
Procure a small quantity of soft, thin sheet iron and cut out a sufficient number of rectangular pieces, 3 in. by 4-1/4 in., to make a pile 3/4 in. in height when firmly pressed together. Now cut a rectangular notch in each of these pieces, 2 in. wide and 3-5/8 in. long. The sides of this notch can be cut with a pair of tinner's shears, and the end can be cut with a sharp cold-chisel. Be careful not to bend either piece any more than you can help. The outside piece, or the one in which the notch is cut, should have dimensions corresponding to those given in Fig. 12. When all of these pieces have been cut, as indicated above, the rectangular pieces, 2 in. by 3-5/8 in., that were cut out to form the notch in the larger pieces, should have two of their corners cut away, so as to form pieces whose dimensions correspond to those given in Fig. 13. These last pieces are to form the core and part of the end of the transformer. Now make sure that all the edges of the pieces are perfectly smooth and that they are all of the same size; then give each one a coat of very thin shellac.
Fig. 12—Outer Portion of the Magnetic Circuit
Fig. 13—Inner Portion of the Magnetic Circuit
Now cut from a piece of insulating fiber, that is about 1/16 in. thick, two pieces whose dimensions correspond to those given in Fig. 14. When these pieces are completed, the core of the transformer can be assembled as follows: Place the T-shaped pieces, whose dimensions correspond to those given in Fig. 13, through the openings in the pieces of insulation, alternate pieces being put through the openings from opposite sides. The distance from outside to outside of the pieces of insulation should be exactly the same as the length of the vertical portion of the T-shaped pieces forming the core, or 3 in.
Fig. 14—Insulating Washer
Cut from some soft wood four pieces having cross sections whose dimensions correspond to those given in Fig. 15, and of such a length that they will just slip down between the two pieces of insulation. These pieces should now be placed on the four sides of the iron core and covered with several layers of heavy insulating cloth. Each layer of the cloth should be shellacked as it is put on, which will increase the insulation and at the same time help in holding the wooden pieces in place. You are now ready to start winding the transformer.