Fig. 178. ASTATIC GALVANOMETER.
Galvanometer, Astatic.
A galvanometer with a pair of magnetic needles connected astatically, or
parallel with their poles in opposition. (See Astatic Needle.) Each
needle has its own coil, the coils being wound in opposite directions so
as to unite in producing deflections in the same sense. As there should
be some directive tendency this is obtained by one of the magnets being
slightly stronger than the other or by the proximity of a fixed and
adjustable controlling magnet, placed nearer one needle than the other.
For small deflections the currents producing them are proportional to
their extent.
Galvanometer, Ballistic.
A galvanometer whose deflected element has considerable moment of
inertia; the exact opposite of an aperiodic or dead beat galvanometer.
(See Galvanometer, Aperiodic.) All damping by air vanes or otherwise
must be carefully done away with.
Fig. 179. SIEMENS & HALSKE'S GALVANOMETER.
Siemens & Halske's galvanometer is of the reflecting or mirror type (see
Galvanometer, Reflecting) with suspended, bell-shaped magnet, in place
of the ordinary magnetic needle, or astatic combination of the lightest
possible weight in the regular instrument. A copper ball drilled out to
admit the magnet is used as damper in the ordinary use of the
instrument. To convert it into a ballistic galvanometer the copper ball
is removed. The heavy suspended magnet then by its inertia introduces
the desired element into the instrument.