*The Coil Heads* are made from two hardwood blocks three inches square and one-half an inch thick. A circular groove, two and one-half inches in diameter and one-quarter of an inch deep is turned in each block to receive a cardboard tube of the same diameter. The two upper corners of each of the blocks are rounded slightly to improve their appearance. One of the heads has a round hole one-half inch in diameter bored through the centre so that one end of the core may be passed through. The core should project through about one-eighth of an inch. The other head is bored with a hole of the same diameter but the hole should not pass all the way through, and serves merely to receive and support the other end of the core. [Illustration: FIG. 114.—Details of the Wooden Base.] ─────────────────────────────────────────────────────────────────── Size of Coil A B C ─────────────────────────────────────────────────────────────────── 1/4 in. 8 in. 4 in. 1 1/8 in. ─────────────────────────────────────────────────────────────────── 1/2 in. 8 1/4 in. 4 1/2 in. 1 1/8 in. ─────────────────────────────────────────────────────────────────── 1 in. 10 1/4 in. 4 1/2 in. 1 1/4 in. ─────────────────────────────────────────────────────────────────── *The Base* of the coil is a box eight inches long, four inches wide and one and one-eighth inches deep. It is made in the form of a hollow box. Give the base a coat of stain, preferably some dark color, such as mahogany, and then a coat of shellac or varnish. The wooden heads should be stained black. The parts of the interrupter are shown in Figure 115. They cannot be made by every experimenter on account of the tools required, but can be purchased very cheaply from concerns dealing in material for experimenters. *The Standard* is a piece of hexagonal brass rod one and three-sixteenths inches long and three-eighths of an inch in diameter. The lower end of the standard is tapped to receive an 8-32 machine screw. A hole is bored at right angles to the axis of the standard, one inch from the lower end and tapped to receive an 8-32 thumbscrew. [Illustration: FIG. 115.—Details of the Interrupter. The Spring and Standard for the One inch coil should be made one-quarter of an inch longer.] The Thumbscrew should be about seven-eighths of an inch long and be provided with a lock nut so that it can be fastened in position. The end of the thumb-screw is drilled with a small hole and a contact point driven in tightly. The contact point is usually a piece of platinum wire, but in the case of a small coil may be a piece of the special contact wire which is sold for just this purpose. [Illustration: FIG. 116.—The tube.]