When you are designing electromagnets for use with alternating
currents, it is necessary to make a change in one respect, namely, you
must so laminate the iron that internal eddy currents shall not occur;
indeed, for all rapid-acting electromagnetic apparatus it is a good
rule that the iron must not be solid. It is not usual with telegraphic
instruments to laminate them by making up the core of bundles of iron
plates or wires, but they are often made with tubular cores, that is
to say, the cylindrical iron core is drilled with a hole down the
middle, and the tube so formed is slit with a saw cut to prevent the
circulation of currents in the substance of the tube. Now when
electromagnets are to be employed with rapidly alternating currents,
such as are used for electric lighting, the frequency of the
alternations being usually about 100 periods per second, slitting the
cores is insufficient to guard against eddy currents; nothing short of
completely laminating the cores is a satisfactory remedy. I have here,
thanks to the Brush Electric Engineering Company, an electromagnet of
the special form that is used in the Brush arc lamp when required for
the purpose of working in an alternating current circuit. 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. The eddy currents set up by induction in neighboring masses
of metal, especially in good conducting metals such as copper, give
rise to many curious phenomena. For example, a copper disk or copper
ring placed over the pole of a straight electromagnet so excited is
violently repelled. These remarkable phenomena have been recently
investigated by Professor Elihu Thomson, with whose beautiful and
elaborate researches we have lately been made conversant in the pages
of the technical journals. He rightly attributes many of the repulsion
phenomena to the lag in phase of the alternating currents thus induced
in the conducting metal. The electromagnetic inertia, or
self-inductive property of the electric circuit, causes the currents
to rise and fall later in time than the electromotive forces by which
they are occasioned. In all such cases the impedance which the circuit
offers is made up of two things--resistance and inductance. Both these
causes tend to diminish the amount of current that flows, and the
inductance also tends to delay the flow.