The Standard Electrical Dictionary / A Popular Dictionary of Words and Terms Used in the Practice of Electrical Engineering

by T. O'Conor Sloane

Fig. 181. DIFFERENTIAL GALVANOMETER.
Galvanometer, Direct Reading.
A calibrated galvanometer, whose scale is graduated by volts or amperes,
instead of degrees.
Galvanometer, Marine. (Sir William Thomson's.)
A galvanometer of the reflecting type, for use on shipboard. A fibre
suspension is adopted for the needle. The fibre is attached to a fixed
support at one end and to a spring at the other, and the needle is
suspended by its centre of gravity. This secures it to a considerable
extent from disturbance due to the rolling of the ship. A thick iron box
encloses the needle, etc., to cut off any magnetic action from the ship.
(See Galvanometer, Reflecting.)
Galvanometer, Potential.
A galvanometer wound with fine German silver wire to secure high
resistance used for determination of potential difference.
Galvanometer, Proportional.
A galvanometer so constructed that the deflections of its index are
proportional to the current passing. It is made by causing the
deflecting force to increase as the needle is deflected, more and more,
or by causing the restitutive force to diminish under like conditions,
or by both. The condition is obtained in some cases by the shape and
position of the deflecting coils.
Galvanometer, Quantity.
A galvanometer for determining quantities of electricity, by the
deflections produced by discharging the quantities through their coils.
It is a ballistic galvanometer with very little or no damping.
270 STANDARD ELECTRICAL DICTIONARY.

Fig. 182. PRINCIPLE OF REFLECTING GALVANOMETER.

Fig. 183. REFLECTING GALVANOMETER.
Galvanometer, Reflecting.
A galvanometer the deflections of whose needle are read by an image
projected by light reflected from a mirror attached to the needle or to
a vertical wire carrying the needle. A lamp is placed in front of the
instrument facing the mirror. The light of the lamp is reflected by the
mirror upon a horizontal scale above the lamp. An image of a slit or of
a wire may be caused thus to fall upon the scale, the mirror being
slightly convex, or a lens being used to produce the projection.
271 STANDARD ELECTRICAL DICTIONARY.
If the mirror swings through a horizontal arc, the reflected image will
move, in virtue of a simple geometrical principle, through an arc of
twice as many degrees. The scale can be placed far from the mirror, so
that the ray of light will represent a weightless index of very great
length, and minute deflections of the needle will be shown distinctly
upon the scale.
In the cut, Fig. 182, the ray of light from the lamp passes through the
aperture, m m, and is made parallel by the lens, L. At s is the mirror
attached to the needle and moving with it. A scale placed at t receives
the reflection from the mirror. The cut, Fig. 183, shows one form of the
instrument set up for use.
Synonym--Mirror Galvanometer.
Galvanometer Shunt.
To prevent too much current passing through a galvanometer (for fear of
injury to its insulation) a shunt is sometimes placed in parallel with
it. The total current will be distributed between galvanometer and shunt
in the inverse ratio of their respective resistances. (See Multiplying
Power of a Shunt.)
272 STANDARD ELECTRICAL DICTIONARY.

Fig. 184. SINE GALVANOMETER.
Galvanometer, Sine.
A galvanometer whose measurements depend upon the sine of the angle of
deflection produced when the coil and needle lie in the same vertical
plane.
The needle, which may be a long one, is surrounded by a coil, which can
be rotated about a vertical axis passing through the point of suspension
of the needle. Starting with the needle at rest in the plane of the
coil, a current is passed through the coil deflecting the needle, the
coil is swung around deflecting the needle still more, until the needle
lies in the plane of the coil; the intensity of the current will then be
in proportion to the sine of the angle through which the coil and needle
move.
In the galvanometer M is a circle carrying the coil, N is a scale over
which the needles, m and n, move, the former being a magnetic needle,
the latter an index at right angles and attached thereto; a and b are
wires carrying the current to be measured. The circles, M and N, are
carried by a base, O, around which they rotate. H is a fixed horizontal
graduated circle. In use the circle, M, is placed in the magnetic
meridian, the current is passed through the coil, M; the needle is
deflected; M is turned until its plane coincides with the direction of
the needle, m. The current strength is proportional to the sine of the
angle of deflection. This angle is measured by the vernier, C, on the
circle, H. The knob, A, is used to turn the circle, M.
273 STANDARD ELECTRICAL DICTIONARY.

Fig. 185. TANGENT GALVANOMETER.
Galvanometer, Tangent.
A galvanometer in which the tangents of the angles of deflection are
proportional to the currents producing such deflections.
For this law to apply the instrument in general must fulfill the
following conditions:
(1) The needle must be controlled by a uniform magnetic field such as
that of the earth;
(2) the diameter of the coil must be large compared to the length of the
needle;
(3) the centre of suspension of the needle must be at the centre of the
coil;
(4) the magnetic axis of the needle must lie in the plane of the coil
when no current is passing.
If a single current strength is to be measured the best results will be
attained when the deflection is 45°; in comparing two currents the best
results will be attained when the deflections as nearly as possible are
at equal distances on both sides of 45°.
The needle should not exceed in length one-tenth the diameter of the
coil.
For very small deflections any galvanometer follows the law of
tangential deflection.
As for very small deflections the tangents are practically equal to the
arcs subtended, for such deflections the currents are proportional to
the deflections they produce.
The sensibility is directly proportional to the number of convolutions
of wire and inversely proportional to their diameter.
The tangent law is most accurately fulfilled when the depth of the coil
in the radial direction is to the breadth in the axial direction as
squareRoot(3):squareRoot(2), or about as 11:9.
Galvanometer, Torsion.
A galvanometer whose needle is suspended by a long filament or by a
thread and spiral spring against whose force of torsion the movements of
the needle are produced. The current strength is determined by bringing
the needle back to its position of rest by turning a hand-button or
other arrangement. The angle through which this is turned gives the
angle of torsion. From this the current strength is calculated on the
general basis that it is proportional to the angle of torsion.