*The Contact* wire should be about three-thirty-seconds of an inch long and be slipped in the hole and riveted in place.
*The Armature* is a piece of soft iron cut out of a rod one-half an inch in diameter. It should be about one-quarter of an inch thick, but should have a small stud left on one face so that the stud may be slipped into the hole marked C in the steel spring and the armature then riveted firmly to the spring by flattening out the stud.
[Illustration: FIG. 117.—The Bridge.]
────────────────────────────────────────────────────────────────── Size of Coil E Length I Width ────────────────────────────────────────────────────────────────── 1/4 inch 4 7/8 inch 3/4 inch ────────────────────────────────────────────────────────────────── 1/2 inch 5 1/8 inch 3/4 inch ────────────────────────────────────────────────────────────────── 1 inch 7 1/8 inch 3/4 inch ──────────────────────────────────────────────────────────────────
The spring is bent at right angles about one-half inch from the end as shown by the dotted line in Figure 115.
*A Cardboard Tube*, two and one-half inches in diameter and four and one-quarter inches long, is used to inclose the primary and secondary windings when they are in position. The tube may be given a very fine appearance by gluing a single layer of black bookbinders’ cloth over it.
*The Bridge* shown in Figure 117 is a strip of wood about one-eighth of an inch thick, four and seven-eighths inches long and three-quarters of an inch wide. It is stained to match the base and is then given a coat of shellac or varnish. Two holes are bored in, the bridge, one inch from each end.
The coil is now ready for assembling. Figure 118. shows a cross-section of the complete coil.
Slip the secondary over the insulating cloth or paper wrapped around the primary. Pass one end of the core through the hole in the coil head making sure that the groove is on the side towards the secondary. The secondary should be exactly in the centre of the primary.
[Illustration: FIG. 118.—Section of the Spark Coil showing the arrangement of the Parts.]