[Illustration:  Geissler Tube]

How to Make a Jump-Spark Coil [113]

The induction coil is probably the most popular piece of apparatus in the electrical laboratory, and particularly is it popular because of its use in experimental wireless telegraphy.  Ten years ago wireless telegraphy was a dream of scientists; 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.