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Show Form Wound Coi'i Used by Amateurs Method of Procedure Is Outlined Out-lined for Electrical Workers. By S. E. WATSON Every amateur electrical worker has occasion at various times to use form-wound form-wound coils of special dimensions. In the construction of plain magnets or open-core apparatus the wire is usually usual-ly wound In place directly upon the core. With dynamo or motor fields and closed-core apparatus it Is usually Impracticable and in seme cases impossible im-possible to wind the wire in this way. Consequently the colls must first be wound upon a form. For the average job a form should be made of exactly the same length as Is desired for the finished coll, ninklng due allowance for the necessnry insulation insu-lation to be added later. Each of the other two dimensions should be about one-eighth inch greater than the corresponding cor-responding dimensions of the core upon which the coil Is to be placed. A block of wood planed to the oversize over-size dimensions indicated above and snwed to the desired length Is the most convenient kind of core form. A quarter-Inch hole is bored through the longitudinal axis of this block and receives the axle used In rotating rotat-ing the form. The appearance of the finished block Is shown In Fig. 1. Above Illustrations Show Method of Constructing Form and Means of Winding Coll. Two end pieces of suitable size are made from this board, and a quarter-Inch quarter-Inch hole Is bored In the center of each. One end piece Is screwed to each end of the core, Its center hole coinciding with that in the core. The spool thus formed Is slipped onto a quarter-Inch rod which has one end threaded for several Inches and the other bent In the form of a crank. A tap is screwed up tightly to each end of the spool, Mid the whole Is mounted mount-ed as shown in Fig. 2. The bearings are simply wooden uprights with hole for the axle. If all work has been done with reasonable accuracy the spool should show no tendency to wobble when the crank is turned. The first step before winding the coil Is to be sure that It will slip off the core readily when it fs finished. Wind on the core a single layer of small hard-twisted cord. This layer must be wound perfectly smooth, and the ends should be brought out through small holes In the end pieces. Wrap upon this layer two or three thicknesses of thin, stiff paper, Just wide enough to come flush against both ends. Glue both tabs of the paper pa-per slightly, just enough to hold It In place. On each side of the core and parallel paral-lel to the axis glue a strip of narrow cloth tape, as shown at A, Fig. 2, allowing al-lowing each end to come up from the core along the end pieces, and fasten temporarily upon the outside. The tape ends must be of sufficient length to tie over the coll when It is wound, thus providing a method of binding it tightly In the process of removal. If the coil Is to be a large one, two or more pieces of tape should be placed on each side of the form. After these binders are In place paint the core with shellac or Insulating paint. Pass one end of the wire to be wound through a small hole drilled through one end-piece. Draw several Inches of wire through and fasten securely by wrapping around the shaft. Wind the required number of turns on the spool, laying each turn as closely as possible against the preceding pre-ceding one. Paint each layer, as it Is wound, with a liberal coat of insulating insulat-ing paint and allow It to partly dry before putting on the next layer. Wire especially enameled wire may be placed much more satisfactorily If It Is wound on a bed of paint which has been allowed to dry just to the point of tackiness. When all the winding Is in place paint the last layer, lay-er, release the tab ends and tie each piece of tape firmly across the coll, as shown In Fig. 3. Allow time for the paint to harden before removing the coll. Passing Just sufficient current cur-rent through the coll to warm up the wire will hasten the drying. Take the form from the axle, remove re-move one of the heads and grasp the end of the layer of string. By pulling pull-ing in the direction parallel to the axis the string may be removed, thus leaving a free space between the coll and the core. Remove the coll from the core, tape It and It Is finished. In the final taping the binding tabs should be removed, but It is not necessary nec-essary to remove the paper. Narrow tape is best and ma"kes a much neater job on small colls. Each turn should overlap the preceding one by half Its width. Going once around the col', with tape in this way gives two thicknesses thick-nesses at every point, and tills is usually sufficient. t Science and Invention and Radio News.) |