*The Regulator Tube* should be two and seven-sixteenths inches long and have an inside diameter of five-sixteenths of an inch. In case the experimenter has any difficulty in securing a tube of this size he can easily make one by rolling a piece of sheet brass around a suitable sized rod and soldering the edges. When the tube is slipped over the core and moved in and out of the bobbin, after the coil has been finished and set in operation, it will regulate the strength of the secondary or "shocking" current. When the tube is completely out of the bobbin, the current will be the strongest. If a paper scale, divided into sixteenths of an inch and bearing numbers, is pasted around the tube it will act as an indicator of the current strength. The base is wood and measures four and five-eighths inches long, two and three-quarters inches wide and is one-half an inch thick. The proper location of the various holes can be found by referring to the illustration in Figure 103. The corners of the base should be slightly rounded so as to improve the appearance. Figure 104 is a top view of the coil after it has been assembled. The bobbin is mounted a little back of the centre of the base so that the core cap will be opposite the armature of the vibrator. [Illustration: FIG. 102.—Regulator Tube.] *The Vibrator Parts* are illustrated in Figures 101 and 106. The vibrator spring is a piece of thin steel or phosphor bronze, one-quarter of an inch wide and one and three-sixteenths inches long. A soft iron armature, five-sixteenths of an inch in diameter and one-eighth of an inch thick is fastened to one end of the spring. A five-thirty-seconds inch hole is drilled in the other end and it is then mounted in the "vibrator posts". This consists of a small brass pillar, one-quarter of an inch in diameter and five-eighths of an inch long. The lower end is drilled, and tapped to receive a 4-36 screw which fastens it to the base. A saw slot one-quarter of an inch deep is made in the top to receive the vibrator spring. The post which holds the adjustment screw is identical with that which supports the vibrator spring, except that a threaded hole is drilled through the saw slot, fifteen-thirty-seconds of an inch from the bottom, through which the screw passes. The end of the adjustment screw, which may be an ordinary knurled thumbscrew from a binding post, should be fitted with a "contact" point. The best contact point is a piece of platinum, but a small piece of silver can be made to serve the purpose very well. [Illustration: FIG. 103.—The Base with Location of Holes.] [Illustration: FIG. 104.—Top View of Finished Coil.] The vibrator spring should also be fitted with a similar contact point. [Illustration: FIG. 105.—Side View of Completed Coil.]