Provide as little copper as you conveniently can,
sacrificing economy in that case to the attainment of your object;
but, of course, you must use fireproof material, such as asbestos, for
insulating, instead of cotton or silk.
A USEFUL GUIDING PRINCIPLE.
In all cases of design there is one leading principle which will be
found of great assistance, namely, that a magnet always tends so to
act as though it tried to diminish the length of its magnetic circuit.
It tries to grow more compact. This is the reverse of that which holds
good with an electric current. The electric circuit always tries to
enlarge itself, so as to inclose as much space as possible, but the
magnetic circuit always tries to make itself as compact as possible.
Armatures are drawn in as near as can be, to close up the magnetic
circuit. Many two-pole electromagnets show a tendency to bend together
when the current is turned on. One form in particular, which was
devised by Ruhmkorff for the purpose of repeating Faraday's celebrated
experiment on the magnetic rotation of polarized light, is liable to
this defect. Indeed, this form of electromagnet is often designed very
badly, the yoke being too thin, both mechanically and magnetically,
for the purpose which it has to fulfill.
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