These points are of vital importance in the action of dynamo electric
generators. It remains, however, yet to be shown how these transient and
momentary induction currents can be so directed and manipulated as to be
made to combine into a steady and continuous supply. 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.


Dr. Odling delivered a lecture on the above before the Chemical Society,