[Illustration: FIG. 100.—Construction of the Core.]
The end of the core is slipped into an iron "core" cap. This is shown in Figure 101. It consists of a small cylinder, seven-sixteenths of an inch in length, cut from a piece of iron rod, three-eighths of an inch in diameter. A hole nine-thirty-seconds of an inch in diameter and five-sixteenths of an inch deep should be drilled in one end. This cap is slipped over one end of the core. It should fit very tightly so that it will not pull out.
The core is then slipped into the bobbin with the outside end of the core cap projecting about one-sixteenth of an inch out of the end at which the primary terminals lead out. The cap should fit into the bobbin very snugly so that the core is held firmly in place.
Since the hole through the bobbin is three-eighths of an inch in diameter and the core is only nine-thirty-seconds of an inch in diameter there should be a space all around the core, except at the end where the cap is, so that a thin brass "regulator" tube can be slipped in and out over the tube.
[Illustration: FIG. 101.—Vibrator Parts and Core Cap.]
*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.