Fig. 177. ELECTRIC FUSE.
Fuse, Electric.
A fuse for igniting an explosive by electricity. There are two kinds. In
one a thin wire unites the ends of the two conducting wires as they
enter the case of the fuse. The larger wires are secured to the case, so
that no strain comes on the fine wire. On passing a current of
sufficient strength the small wire is heated. In use the fuse is bedded
in powder, which again may be surrounded by fulminating powder, all
contained in a copper or other metallic case. Such a detonator is used
for exploding guncotton and other high explosives.
The other kind of fuse is similar, but has no thin connecting wire. The
ends of the conductors are brought nearer together without touching. In
use a static discharge is produced across from end to end of the
conductors, igniting a proper explosive placed there as in the other
case.
The first kind of fuse is generally operated by a battery or small
mechanical generator--the latter by a spark coil, frictional or
influence machine or by a Leyden jar.

264 STANDARD ELECTRICAL DICTIONARY.
Galvanic. adj.
Voltaic; relating to current electricity or the electrolytic and
electro-chemical relations of metals. (For titles in general under this
head see Voltaic--or the main title.)
Galvanic Element.
A galvanic couple with exciting fluid and adjuncts; a galvanic cell. The
word element is sometimes applied to the electrodes of a cell, as the
carbon element or zinc element.
265 STANDARD ELECTRICAL DICTIONARY.
Galvanic Polarization.
The polarization of a voltaic couple. (See Polarization.)
Galvanism.
The science of voltaic or current electricity.
Galvanization.
(a) Electroplating or depositing a metal over the surface of another by
electrolysis.
(b) In medical electricity the effects produced on any part of the
system by the current of voltaic battery. Various descriptive
qualifications are prefixed, such as "general" galvanization, indicating
its application as applied to the whole body, "local" for the reverse
case, and so on.
Galvanization, Labile.
Application of the galvanic current in electro-therapeutics where one
sponge electrode is employed which is rubbed or moved over the body, the
other being in constant contact with the body.
Galvanized Iron.
Iron coated with zinc by cleaning and immersion in melted zinc. The iron
is prevented from rusting by galvanic action. It forms the negative
element in a couple of which the zinc is the positive element. From this
electric protective action the name is derived.
Galvano-cautery, Chemical.
Electro-therapeutic treatment with sharp electrodes, one of which is
inserted in the tissue and a current passed by completing the circuit
through the tissue so as to electrolyze or decompose the fluids of the
tissue. It is applied in the removal of hair or extirpation of the
follicle. The process is not one of heating, and is improperly named
cautery.
Galvano-faradization.
In medical electricity the application of the voltaic and induced or
secondary current simultaneously to any part of the system.
Galvanometer.
An instrument for measuring current strength and sometimes for measuring
inferentially potential difference, depending on the action of a
magnetic field established by the current, such action being exerted on
a magnetic needle or its equivalent.
A current passing through a conductor establishes circular lines of
force. A magnetic needle placed in their field is acted on and tends to
place itself parallel with the lines, in accordance with the principles
of current induction. (See Induction, Electro-magnetic.) A common
compass held near a conductor through which a current is passing tends
to place itself at right angles to such conductor. For a maximum effect
the conductor or the part nearest the needle should lie in the magnetic
meridian. If at right angles thereto its action will only strengthen the
directive force of the earth's induction or magnetic field, as the
needle naturally points north and south. Such combination is virtually a
galvanometer.
266 STANDARD ELECTRICAL DICTIONARY.
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.
267 STANDARD ELECTRICAL DICTIONARY.

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.
268 STANDARD ELECTRICAL DICTIONARY.
Referring to the cut, Fig. 179, M is the suspended magnet, with north
and south poles n and s; S is the reflecting mirror; r is the tube
containing the suspending thread; R is the damper removed for ballistic
work.
The ballistic galvanometer is used to measure quantities of electricity
in an instantaneous discharge, which discharge should be completed
before the heavy needle begins to move. The extreme elongation or throw
of the needle is observed, and depends (1) on the number of coulombs (K)
that pass during the discharge; (2) on the moment of inertia of the
needle and attached parts; (3) on the moment of the controlling forces,
i. e., the forces tending to pull the needle back to zero; (4) on the
moment of the damping forces; (5) on the moment of the deflecting forces
due to a given constant current. The formula is thus expressed:
K = (P / PI ) * A * sin( kº / 2 ) / tan( aº )
in which K = coulombs discharged; P = periodic time of vibration of
needle; A = amperes producing a steady deflection equal to aº ; kº =
first angular deflection of needle. For accuracy kº and aº should both
be small and the damping so slight as to be negligible. Otherwise a
correction for the latter must be applied. For approximate work for kº
and aº the deflections read on the scale may be used with the following
formula:
K = (P / PI ) * ( A / 2 ) * ( kº / aº )
Galvanometer Constant.
Assume a galvanometer with a very short needle and so placed with
respect to its coils that the magnetic field produced by a current
circulating in them is sensibly uniform in the neighborhood of the
needle, with its lines of force at right angles thereto. The field is
proportional to the current i, so that it may be denoted by G i. Then G
is the galvanometer constant. If now the angle of deflection of the
needle is ? against the earth's field H, M being the magnetic moment of
the needle we have G i M cos ? = H M sin ? or i = (H/G)* tan ?. H/G is
the reduction factor; variable as H varies for different places.
For a tangent galvanometer the constant G is equal to 2*PI*(n/a), in
which n denotes the number of turns of wire, and a denotes the radius of
the circle.
Galvanometer, Differential.
A galvanometer in which the needle is acted on by two coils wound in
opposition, each of equal deflecting action and of equal resistance. If
a current is divided between two branches or parallel conductors, each
including one of the coils, when the needle points to zero the
resistances of the two branches will bc equal. In the cut, C C'
represent the coils, and A and B the two leads into which the circuit, P
Q, is divided.
269 STANDARD ELECTRICAL DICTIONARY.

Fig. 180. THEORY OF DIFFERENTIAL GALVANOMETER.