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.
ELECTROMAGNETS FOR QUICKEST ACTION.
I have already mentioned Hughes' researches on the form of
electromagnet best adapted for rapid signaling.
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