ELECTROMAGNETS FOR USE WITH ALTERNATING CURRENTS.
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.
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