Plumb Line

plumb line

a string with a metal weight at one end that, when suspended, points directly towards the earth's centre of gravity and so is used to determine verticality, the depth of water, etc.

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plumb line [′pləm ‚līn]

(engineering)

The string on which a plumb bob hangs.

(geophysics)

A continuous curve to which the direction of gravity is everywhere tangential.

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plumb line

A cord or line, having a metal bob or weight attached to one end, which indicates the true vertical direction.

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Plumb Line 

a line that coincides with the direction of gravity at any given point in space. The direction of a plumb line coincides with the direction of a string to which a freely suspended weight, or plumb bob, is attached. This direction is everywhere perpendicular to surfaces that are equipotential with respect to gravitational force. The equipotential surfaces of the earth are not parallel to each other because of the earth’s spheroidicity and irregular internal structure. Therefore, infinitely small consecutive segments of a plumb line, as normals to infinitely close equipotential surfaces of the earth, form a curve of double curvature, which is known as a line of force of the earth’s gravitational field. In other words, the plumb line at a given point is tangential to the line of force at this point.

The direction of a plumb line at a given point of the earth’s surface relative to the earth’s equatorial planes and prime meridian can be found by determining the latitude and longitude of this point from astronomical observations. Analysis of the directional changes of a plumb line and of the line’s deviations from the normal to the surface of the earth ellipsoid at different points of the earth’s surface makes it possible to determine the shape and dimensions of the earth and to obtain indirect data on its internal structure (seeDEFLECTION OF THE PLUMB LINE). The direction of a plumb line fluctuates as a result of the redistribution of masses within the earth and on its surface. Owing to the attraction of the moon and the sun, the plumb line sustains periodic variations of 0.025”.

A. A. IZOTOV