Every time that the interrupter shuts off the battery current in the primary, currents are induced in the secondary which are of sufficiently high voltage or pressure to leap across a space in the shape of sparks.
A spark coil is not very hard to construct, but it requires careful workmanship and patience. It is not usually a job which can be finished in a day, but will take quite a little time, especially in winding the secondary.
In describing the construction of a spark coil in this chapter, directions and dimensions have been given for a one-quarter inch coil. The general method of procedure is the same for all sizes of coils, the dimensions only, are different. The experimenter may therefore easily build a larger coil than the one-quarter inch by consulting the tables of dimensions.
Parts for spark coils are for sale by many electrical houses and it is possible to purchase the complete parts for less than the separate materials would cost.
Such parts are made by special machinery and therefore are produced quite cheaply.
The Core is made of soft iron "core wire" about No. 20 or 22 B. & S. Gauge. Each piece should be four and three-quarters inches long. Iron wire may be bought already cut to length from various houses dealing in experimenter’s supplies. Considering the amount of labor that is required to cut each piece to length arid then straighten it out, it is cheaper to purchase the wire already cut.
[Illustration: FIG. 110.—The Primary and Core.]
────────────────────────────────────────────────────────────────── Size of Coil. O Length of P Diameter of Primary Wire Core Core ────────────────────────────────────────────────────────────────── 1/4 inch 4 3/4 inches 1/2 inch 18 B. & S. ────────────────────────────────────────────────────────────────── 1/2 inch 5 inches 5/8 inch 18 B. & S. ────────────────────────────────────────────────────────────────── 1 inch 7 inches 5/8 inch 18 B. & S. ──────────────────────────────────────────────────────────────────
"Core wire" is a special grade of wire which is very soft and has been annealed.
If ordinary iron wire is used in making a core, it must first be annealed. This is accomplished by tying the wire in a compact bundle and placing it in a wood fire until it is red hot. After the wire is hot, cover it with ashes and allow the fire to die away.