# plumb line

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## 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.

## 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

## 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.

## plumb line

A cord or line, having a metal bob or weight attached to one end, which indicates the true vertical direction.
References in periodicals archive ?
Take a moment and reflect on your own plumb line indications.
Observations, made at different mount rotation azimuths A, gives series of plumb line and ellipsoid's normal positions in rotating coordinate systems of imager and tiltmeter.
The effect induced by density contrast [DELTA][rho] (approximately -0.25 Mg/[m.sup.3]) is a cause of gravity changes and changes in direction of plumb line at Inowroclaw.
[bar.[gamma] is the mean reference gravity along the plumb line,
* Self-leveling laser plumb lines are available that project a vertical laser line onto any surface.
That plumb line showed how far the people had strayed from the uprightness required of them by God.
When passive end-feel is achieved, the examiner drops an imaginary plumb line perpendicular to the long axis of the tibia from the fifth MTP joint to the plantar-most spot on the heel and takes a reading by visualizing the distance between that spot on the heel and the "plumb line" (Fig.
That step is to develop a personal plumb line. My plumb line is taken and developed from an outside source that is as unchanging as I can find.
Line up the first strip with the plumb line and smooth down the first section along the line, gently pushing paper into the corner and edges of the ceiling.
When turned on, it projects a registration target composed of a vertical plumb line and horizontal leveling line.
Once these are all assembled, take a plumb line up from the base unit to get your fixing position.
If a plumb line is lined up with the stars when they appear on top of one another this will give a true north point.

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