# Torsion Balance

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## torsion balance,

instrument used to measure small forces. It is based on the principle that a wire or thread resists twisting with a force that is proportional to the stress. The torsion balance consists essentially of a wire or thread attached at one end and arranged in such a way that a force applied at the other, or free, end tends to twist it out of shape. The force is measured by the extent to which the wire or thread is so twisted. Torsion balances are used to measure small electric, magnetic, and gravitational forces. One type is used to measure small weights. The invention of the torsion balance is commonly credited to the English geologist John Michell, who made his instrument c.1750, and to the French physicist Charles A. de Coulomb, who independently devised such a balance c.1777.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

## Torsion Balance

an instrument for measuring second derivatives of gravitational potential, which characterize the curvature of the equipotential gravitational surface and the horizontal change (gradient) of the force of gravity. Torsion balances that measure only gravity gradients are called gradiometers.

The torsion balance was invented at the end of the 19th century by the Hungarian physicist L. Eötvös. It consists of a light horizontal or inclined balance beam with masses suspended from or fastened to its ends at different heights. The beam is supported by a fine elastic torsion thread. In a nonhomogeneous gravitational field a gravitational force moment arises and acts on the beam’s masses. The beam rotates until the moment of attractive force equals the torsion moment of the thread. The derivatives of the gravitational potential are determined by the beam’s angle of rotation as the body of the balance is successively placed at various angles to the meridian (at different azimuths). Photographic recording or visual observation is used.

The construction of the balance eliminates the effects of temperature and of magnetic and electrostatic fields. Second derivatives of gravitational potential are measured with an accuracy of ±(1–2) × 10−9 sec−2. Torsion balances are used to study the distrivution of density inhomogeneities of the upper layers of the earth’s crust for purposes of geological prospecting and analysis. Since the readings of a torsion balance also depend on the actions of masses that make up the earth’s surface terrain, it is necessary to have detailed information about the terrain in the immediate vicinity of the place where measurements are made.

M. U. SAGITOV

## Torsion Balance

a sensitive physical instrument for measuring small forces (force moments). The torsion balance was invented in 1784 by C. Coulomb (seeCOULOMB TORSION BALANCE).

The simplest type of torsion balance consists of a vertical thread from which a light balanced arm is suspended. The forces being measured act on the ends of the arm and rotate it in the horizontal plane until they are balanced by the elastic forces of the twisted thread. The torque Mt may be measured from the angle of rotation ϕ of the arm, since ϕ ~ Mtl/GI, where l is the length of the thread, G is the shear modulus of the thread material, and i is the total moment of inertia of the arm and thread. The readout scale of a torsion balance is usually calibrated directly in units of force or force moment. High sensitivity is attained in torsion balances through the use of a sufficiently long thread with a low shear modulus.

Balances with a moving system consisting of a horizontal axle attached by its ends to spiral springs, with an arm for placing the load, are also called torsion balances.

Torsion balances are used to measure mechanical, electrical, magnetic, and gravitational forces and their variations.

### REFERENCES

Shokin, P. F. Gravimetriia. Moscow, 1964. Chapter 4.
Chechernikov, V. I. Magnitnye izmereniia, 2nd ed. Moscow, 1969. Chapter 7.
Braginskii, V. B., and V. I. Panov. “Proverka ekvivalentnosti inertnoi i gravitatsionnoi mass.” Zhurnal’ eksperimental’noi i teoreticheskoi fiziki, 1971, vol. 61, fase. 9, p. 873.

IU. N. DROZHZHIN

## torsion balance

[′tȯr·shən ‚bal·əns]
(engineering)
An instrument, consisting essentially of a straight vertical torsion wire whose upper end is fixed while a horizontal beam is suspended from the lower end; used to measure minute gravitational, electrostatic, or magnetic forces.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
Observations of the astronomical phenomena by torsion balance. Physics of Consciousness and Life, Cosmology and Astrophysics, v.
Precise underground observations of the partial solar eclipse of 1 June 2011 using a Foucault pendulum and a very light torsion balance. International Journal of Astronomy and Astrophysics, 2012, v.
Live coral: Pieces of live Agaricia did not start calcifying until about 24 hours after collection but if newly collected pieces were suspended on the torsion balance overnight in the dark calcification usually started before morning.
Waterpiked coral: Freshly waterpiked pieces of coral suspended in seawater on the torsion balance were found to have an astonishing initial calcification rate (n=30) more than an order of magnitude higher than the same piece of coral when alive.
It is not quite clear the reason why EPF, and also Eotvos |1891~, used this kind of torsion balance, because it involves some difficulties.
At this point, the problem was to find an expression which included |Omega~, that is, the unknown entity, and then to 'translate' it into an expression that included only quantities which were empirically measured by the torsion balance.
Various techniques not involving a torsion balance have also been used to measure G and other gravitational effects.
In basic form, a torsion balance consists of two weights connected by a horizontal rod (a dumbbell) suspended by a long, thin fiber.
When we consider the meatus facing the test mass of a gravitational torsion balance placed in a vacuum chamber, the very little air depression within the meatus originates a disturbing force on the test mass, which adds to the gravitational force.
This fact agrees with the author conclusions [9] that the torsion balance configuration would have an inherent accuracy of about 10 ppm in determining G, but the uncertainty in the fundamental period reduces this accuracy to 1400 ppm.
In particular, for instance, if a torsion balance registered the repulsing/attracting wave pressure [[??].sub.N] derived from subatomic excitation/relaxation processes, we will have obtained the numerical value of the energy-momentum constant [micro] for time density fields.
A torsion balance registered such forces at ~ [10.sup.-5] dynes in prior experiments.

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