Spool.
This should be made up of a fibre tube 9 inches in length by about 1/16 inch thick, and should be firmly fixed into the spool ends. If it be glued in it should also be pinned as well; it is easily possible to drive in a few screws passing through the tube into the spool ends, particularly as the soft iron core, being of loose wires, will adapt itself to the slightly projecting screw-heads. Remember that this spool must be made strong; if it comes apart during the winding process, much trouble will ensue, and perhaps all the wires lost or ruined. For reasons to be seen later, do not affix the right-hand spool end yet, but have it ready. The core consists of as many fine iron wires, say of No. 22 B. W. gauge, as can be forced into the tube, but the core can better be added after the windings are all in; that is, in such cases where a rigid spool tube is used.