The average size Ruhmkorff coil, for jump spark work, would be one giving a 2-inch spark, specifications for which are as follows:
Spool—Nine inches long by one inch in diameter. End cheeks 4 inches high by 3 inches wide.
Core—Sufficient soft iron wires, 9 inches long by No. 22 B. W. gauge as will fill the spool tube.
Primary—Two layers No. 14 B. & S. gauge cotton-covered copper wire.
Secondary—Two and one-half pounds No. 36 B. & S. gauge double cotton or silk-covered magnet wire wound in four sections (or more than four sections, if feasible).
Condenser—Seventy sheets tinfoil 4 by 7-1/2 inches; 80 sheets condenser paper 5 by 8 inches.
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.
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.