This is a coil designed with two coils, the high tension-(voltage) type. The older version is just a single coil that remains energized until you are ready to fire it; the breaking of a contact creates sparks.
With two coils the breaking of current is still used but it creates a high voltage in the secondary coil that is directed to the plug.
I found a rather good description WITH a recommended amount and size of wire to use for the primary/secondary type of coil.
Check it:
For a coil to give a half to one inch spark in air the following notes and dimensions will give satisfactory results. The core, of a bundle of well-annealed soft iron wire, should be 7 inches long by 3/4 inch diameter. The wire must be in perfectly straight pieces, and No. 22 B.W.G. in thickness. The core is insulated with linen tape, wound on spirally in three layers, each layer well soaked in shellac varnish. The primary coil is wound directly on the insulated core, and should consist of two layers of No. 18 B.W.G. double cotton-covered wire, the length of this coil being about 6 inches, about half a pound of wire being required.
The insulation between the primary and secondary windings is of vital importance, and should take the form of a vulcanite, or fibre, tube, \ inch thick in the walls, and be a good close fit over the primary winding. At each end of the tube, hard wood, or vulcanite, cheeks are to be fitted to form a bobbin, upon which the secondary is wound. The quantity of wire to be employed for the secondary winding will depend on the length of spark desired. For a 1/2-inch spark, use half a pound; for a 3/4-inch spark, three-quarters of a pound; and for a 1-inch spark, one pound. The gauge will be the same in each case, i.e. No. 36 B.W.Gr. double silk-covered. The wire must be free from kinks, and be tested from time to time to make sure that it is continuous. When fully wound, the whole coil should be soaked in hot paraffin wax, to exclude air and damp, and improve the insulation.
Fig. 19.
An efficient condenser is required, and for the three sizes of coils mentioned above this may consist of fifty, seventy, or ninety pieces of tin foil, each measuring 7 inches by 4 inches. The condenser is to be connected in shunt across the primary contact-breaker terminals as in Figs. 19
