A square block, 1 x 1 x 5-16 inches, is cut out of fiber or a close-grained hard wood and a hole three-eighths of an inch in diameter bored through the center. One end of the tube containing the core is smeared with glue and slipped into the block. The end of the tube is allowed to project through about one-sixteenth of an inch. A second block, in the form of a circle three-quarters of an inch in diameter, one-quarter of an inch thick, and having a three-eighths of an inch hole through the center, is glued on the opposite end. [Illustration: Fig. 154.—Details of Various Parts of a Medical Coil.] After the glue has dried, four small holes are drilled in the square head in the approximate positions shown by Figure 154. Four layers of No. 22 B. & S. gauge magnet wire (it may be either silk or cotton, double or single covered) is wound smoothly and carefully over the core. The terminals are led out of the holes _a_ and _b_. The primary is covered with two or three layers of paper, and then enough secondary wound on to bring the total diameter of the coil to about eleven-sixteenths of an inch. The secondary wire must be much finer than the primary. It is possible to use any size from No. 32 to No. 36 B. & S. gauge and obtain good results. The insulation may be either single silk or single cotton. [Illustration: Fig. 155.—Details of Interrupter for Medical Coil.] The secondary terminals are led out through the holes _c_ and _d_. It is perhaps a wise plan to re-enforce these leads with a heavier piece of wire, because otherwise they are easily broken. The interrupter is a simple arrangement capable of being made in several different ways. The drawing shows an arrangement which can be improved upon by any experimenters who are familiar with a medical coil. I have shown the simplest arrangement, so that all my readers will be able to build it, and those who want to improve it can do so. If a small piece of silver is soldered to the spring and to the contact-point it will give better results. The silver is easily secured by cutting up a ten-cent piece. One terminal of the primary is connected to the interrupter spring and the other to a binding-post. The contact-post is also connected to a binding-post. If a battery is connected to the two binding-posts, the current will flow from one post through the coil to the interrupter spring, through the spring to the contact post, and thence back to the battery, making a complete circuit. As soon as the current flows, however, it produces magnetism which draws the spring away from the contact and breaks the circuit, cutting off the magnetic pull. The spring flies back to the contact but is drawn forward again immediately and repeats the operation continuously at a high rate of speed. [Illustration: Fig. 156.—Completed Medical Coil.]