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.] Slide the cardboard tube over the primary and secondary into the groove. Lead the secondary terminals out through small holes in the cardboard tube at opposite ends so that the wires are separated as far as possible. Slip the other coil head over the end of the core and the tube and then mount the coil on the top of the base. The coil should be in the centre of the base when considered from front to back. It should be to the left of centre, however, when viewed from side to side. The two primary heads should be four inches apart When the coil is assembled. The primary wires are led down close to the wooden head and through the base under the coil. They should be kept as far away from the secondary as is possible. The coil is fastened to the base by a wood screw passing through the base into each of the wooden heads. Figure 122 will show best how the interrupter is arranged. The brass spring carrying the armature at its upper end is mounted in front of the core end projecting through the wooden head. The standard carrying the thumbscrew and the lock nut are mounted directly behind the spring in such a position that the contacts touch each other and line up perfectly. [Illustration: FIG. 119.—End View of the Complete Coil.] ────────────────────────────────────────────────────────────────── Size of Coil B Width of Base D Width of Head ────────────────────────────────────────────────────────────────── 1/4 inch 4 inch 3 inch ────────────────────────────────────────────────────────────────── 1/2 inch 4 inch 3 inch ────────────────────────────────────────────────────────────────── 1 inch 4 1/2 inch 3 1/2 inch ────────────────────────────────────────────────────────────────── A small brass washer is placed between the standard and the base so that it will not "bite" into the wood when the screw is tightened. The standard and the spring are both mounted by 8-32 brass machine screws passing through the base. Two binding posts are mounted on the corners of the base near the interrupter. [Illustration: FIG. 120.—Side View of the Completed Coil.]