To bring a magnet
pole up toward a coil of wire is a process which can only last a very
limited time; and its recession from the coil also cannot furnish a
continuous current since it is a process of limited duration. In the
earliest machines in which the principle of magneto-electric induction
was applied, the currents produced were of this momentary kind,
alternating in direction. Coils of wire fixed to a rotating axis were
moved past the pole of a magnet. While the coil was approaching the
lines of force were increasing, and a momentary inverse current was set
up, which was immediately succeeded by a momentary direct current as the
coil receded from the pole. Such machines on a small scale are still to
be found in opticians' shops for the purpose of giving people shocks. On
a large scale alternate current machines are still employed for certain
purposes in electric lighting, as, for example, for use with the
Jablochkoff candle. Large alternate-current machines have been devised
by Wilde, Gramme, Siemens, De Meritens, and others.--_Engineering_.
* * * * *
ON THE UNIT WEIGHT AND MODE OF CONSTITUTION OF COMPOUNDS.
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