Fig. 18. ARAGO'S DISC.
Arago's Disc.
An apparatus consisting of a disc of copper mounted horizontally, or on
a vertical spindle, and so arranged as to be susceptible of rapid
rotation. Immediately over it, and best with a pane of glass
intervening, a magnetic needle is mounted on a pivot directly over the
axis of the disc. If the disc is rotated the lines of force of the
magnet are cut by it, and consequently currents are produced in the
copper. These currents act upon the needle and cause it to rotate,
although quite disconnected. It is advisable for the needle to be strong
and close to the disc, which should rotate rapidly.
Arc v.
To form a voltaic arc.
Arc, Compound.
A voltaic arc springing across between more than two electrodes.
Arc, Metallic.
The voltaic arc produced between terminals or electrodes of metal. The
characteristics of such arc as contrasted with the more usual arc
between carbon electrodes are its greater length for the same
expenditure of energy, its flaming character and characteristic colors
due to the metals employed. It is sometimes, for the latter reason, used
in spectroscopic investigations.
Arc Micrometer.
A micrometer for measuring the distance between the electrodes of a
voltaic arc.
Arc, Simple.
A voltaic arc produced, as usual, between only two electrodes.