Fig. 157.—Diagram showing Essential Parts of Induction Coil.
It consists of a central iron core surrounded by a coil of heavy wire called the "primary," and by a second outside winding of wire known as the "secondary." The primary is connected to a few cells of battery in series with an interrupter. The interrupter makes and breaks the circuit, i. e., shuts the current on and off repeatedly.
Every time that the current is "made" or broken, a high voltage is induced in the secondary. By means of the condenser connected across the interrupter terminals, the current at "make" is caused to take a considerable fraction of time to grow, while at "break," the cessation is instantaneous. The currents induced in the secondary at break are so powerful that they leap across the space in a brilliant torrent of sparks.
Building a Spark-Coil
Perhaps more attempts are made by experimenters to construct a spark-coil than any other piece of apparatus, and the results are usually poor. A spark-coil is not hard to construct, but it requires careful work and patience. It is not a job to be finished in a day, but time must be liberally expended in its construction. Satisfactory results are easily obtained by any one of ordinary mechanical ability if patience and care are used.
Parts for spark-coils are for sale by many electrical houses, and it is possible to purchase a set of such machine-made parts for less than the separate materials usually cost.
For the benefit of those who might wish to build a larger coil than the one described in the following text, a table showing the dimensions of two other sizes will be found.
Fig. 158.—Empty Paper Tube, and Tube filled with Core Wire preparatory to winding on the Primary.
The core is made of very soft iron wire about No. 20 or 22 B. & S. gauge, cut to exact length. Each piece should be six inches long. Iron wire may be purchased from electrical supply houses already cut to various lengths for twenty cents a pound. In view of the amount of labor required carefully to cut each piece to length and then straighten it out so that it will form a neat bundle, it is cheaper to purchase the wire already cut. Such wire has been annealed, i.e., softened by bringing to a red heat and then cooling slowly. In case the wire is purchased at a plumbing shop or a hardware store it must be annealed before it can be used. This is accomplished by tying the wire in a compact bundle and placing it in a wood fire where it will grow red-hot. When this stage is reached, cover the wire with ashes and allow the fire to die away.