sts; today it is the
plaything of school-boys and thousands of grown-up boys as well.

Divested of nearly all technical phrases, an induction coil may be
briefly described as a step-up transformer of small capacity. It
comprises a core consisting of a cylindrical bundle of soft-iron
wires cut to proper length. By means of two or more layers of No.
14 or No. 16 magnet wire, wound evenly about this core, the bundle
becomes magnetized when the wire terminals are connected to a
source of electricity.

Should we now slip over this electromagnet a paper tube upon which
has been wound with regularity a great and continuous length of
No. 36 magnet wire, it will be found that the lines of force
emanating from the energized core penetrate the new coil-winding
almost as though it were but a part of the surrounding air itself,
and when the battery current is broken rapidly a second electrical
current is said to be induced into the second coil or secondary.

All or any of the parts of an induction coil may be purchased
ready-made, and the first thing to do is to decide which of the
parts the amateur mechanic can make and which would be better to
buy ready-made. If the builder has had no experience in
coilwinding it would probably pay to purchase the secondary coil
ready-wound, as the operation of winding a mile or more of fine
wire is very difficult and tedious, and the results are often
unsatisfactory. In ordering the secondary it is always necessary
to specify the length of spark desired.

The following method of completing a 1-in. coil illustrates the
general details of the work. The same methods and circuits apply
to small and larger coils. The ready-made secondary is in solid
cylindrical form, about 6 in. long and 2-5/8 in. diameter, with a
hole

[Illustration: Jump-Spark Coil]

through the winding 1-1/4 in. in diameter, as shown in Fig. 1. The
secondary will stand considerable handling without fear of injury,
and need not be set into a case until the primary is completed.
The primary is made of fine annealed No. 24 iron wire cut 7 in. or
8 in. in length, as the maker prefers, and bundled to a diameter
of 7/8 in. The wires may be straightened by rolling two or three
at a time between two pieces of hard wood. If the amateur has
difficulty in procuring this wire, the entire core may be
purchased ready-made.

After the core wires are bundled, the core is wrapped with one or
two layers of manila paper. The straighter the