A typical galvanometer comprises a flat coil of wire placed horizontally
within which a magnetic needle is delicately poised, so as to be free to
rotate with the least possible friction. The needle may be supported on
a sharp point like a compass needle, or may be suspended by a long fine
filament. It should be covered by a glass plate and box, or by a glass
shade. Finally a graduated disc may be arranged to show the amount of
deflection of the needle.
In use the apparatus is turned about until the needle, as acted on by
the earth's magnetic field, lies parallel to the direction of the coils
of wire. On passing a current through the coil the needle is deflected,
more or less, according to its strength.
By using exceedingly fine wire, long enough to give high resistance, the
instrument can be used for very high potentials, or is in condition for
use in determining voltage. By using a coil of large wire and low
resistance it can be employed in determining amperage. In either case
the deflection is produced by the current.
The needle is often placed above or below the coil so as only to receive
a portion of its effect, enough for all practical purposes in the
commoner class of instruments.
The galvanometer was invented by Schweigger a short time after Oersted's
discovery, q. v.
Galvanometer, Absolute.
A galvanometer giving absolute readings; properly one whose law of
calibration can be deduced from its construction. Thus the diameter of
the coil, and the constants and position of a magnetic needle suspended
in its field being known, the current intensity required to deflect the
needle a given number of degrees could be calculated.
Galvanometer, Aperiodic.
A galvanometer whose needle is damped (see Damping) as, for instance, by
the proximity of a plate of metal, by an air vane or otherwise, so that
it reaches its reading with hardly any oscillation. A very light needle
and a strong magnetic field also conduce to vibrations of short period
dying out very quickly. Such galvanometers are termed "dead-beat." No
instrument is absolutely dead-beat, only relatively so.