The Standard Electrical Dictionary Part 76

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 pa.s.sing. It is made by causing the deflecting force to increase as the needle is deflected, more and more, or by causing the rest.i.tutive 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, Quant.i.ty.

A galvanometer for determining quant.i.ties of electricity, by the deflections produced by discharging the quant.i.ties through their coils.

It is a ballistic galvanometer with very little or no damping.

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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.

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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 pa.s.ses 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 pa.s.sing 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.)

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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 pa.s.sing through the point of suspension of the needle. Starting with the needle at rest in the plane of the coil, a current is pa.s.sed 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 pa.s.sed 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 k.n.o.b, A, is used to turn the circle, M.

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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 pa.s.sing.

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-b.u.t.ton 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.

Fig. 186. TORSION GALVANOMETER.

274 STANDARD ELECTRICAL DICTIONARY.