long or
more.
--Contributed by Joseph B. Bell, Brooklyn.
[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.
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