Primary Winding.
This consists of two layers of No. 14 B. & S. gauge cotton (or silk) covered copper magnet wire, and should be evenly and tightly laid on. For winding coils, a lathe is a most handy machine, or the spool can be mounted on a spindle and rotated by hand. It is not feasible here to give all details of coil-construction; reference should be made to the many excellent works on the subject. The two ends are brought out through holes in the spool ends, as indicated for the simple primary coil before described. After winding, the wire is to be well basted with melted paraffin wax until it is saturated, any excess being scraped off so as to leave a smooth cylindrical surface for the secondary coil. Half a dozen turns of stout paper or oiled silk is now to be wound on, and enough paraffin wax added to leave an insulation at least one-quarter of an inch around the outside of the winding. The right-hand end of spool, where the end was not attached, will require a little care that the wire does not run off; but, as only two layers are to be wound, it is an easy thing to do.
When the primary coil is finished off, cut three pieces of hard rubber four inches square, with a central hole just big enough that they may be slipped on over the primary coil to form divisions into which the secondary wire goes. These can be fixed equal distances apart by means of removable wooden blocks, which are taken off as each section is wound.