It has two
bobbins that are screwed up against the top of an iron box at the head
of the lamp. The iron slab serves as a kind of yoke to carry the
magnetism across the top. There are no fixed cores In the bobbins,
which are entered by the ends of a pair of yoked plungers. Now in the
ordinary Brush lamp for use with a steady current, the plungers are
simply two round pieces of iron tapped into a common yoke; but for
alternate current working this construction must not be used, and
instead a U-shaped double plunger is used, made up of laminated iron,
riveted together. Of course it is no novelty to use a laminated core;
that device, first used by Joule, and then by Cowper, has been
repatented rather too often during the past fifty years to be
considered as a recent invention.
The alternate rapid reversals of the magnetism in the magnetic field
of an electromagnet, when excited by alternating electric currents,
sets up eddy currents in every piece of undivided metal within range.
All frames, bobbin tubes, bobbin ends, and the like, must be most
carefully slit, otherwise they will overheat. If a domestic flat iron
is placed on the top of the poles of a properly laminated
electromagnet, supplied with alternating currents, the flat iron is
speedily heated up by the eddy currents that are generated internally
within it.
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