Torsion balances

Torsion balances

A force can be measured by measuring the twisting of the torsion wire in a torsion balance.

Physics

Keywords

torsion pendulum, torsion wire, Coulomb, Cavendish, Eötvös, gravitational constant, electrostatic interaction, gravity, gravitation, torque, force, mass, charge, inertial mass, gravitational mass, torsion, balance, telescope, ore deposit, oil deposit, mechanics, twisting, physics

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Scenes

Torsion balances

  • Coulomb´s balance - The torsion balance constructed by Charles Coulomb (1736–1806) is used to measure electrostatic interaction.
  • Cavendish balance - The torsion balance constructed by Henry Cavendish (1731–1810) is used to measure gravitational force.
  • Eötvös balance - The torsion balance constructed by Loránd Eötvös (1848–1919) is used to measure gravitational force. It was used to find oil and ore deposits and to map subterranean features. By use of the balance Eötvös proved that the gravitational mass and the inertial mass of a material are proportional. This is one of the basic assumptions of Einstein’s general theory of relativity.

Coulomb’s balance
The torsion balance constructed by Charles Coulomb (1736–1806) is used to measure electrostatic interaction.

Cavendish balance
The torsion balance constructed by Henry Cavendish (1731–1810) is used to measure gravitational force.

Eötvös balance
The torsion balance constructed by Loránd Eötvös (1848 – 1919) is used to measure gravitational force.
It was used to find oil and ore deposits and to map subterranean features. By use of the balance, Eötvös proved with high accuracy that the gravitational mass and the inertial mass of a material are proportional. This is one of the basic assumptions of Einstein’s general theory of relativity.

Coulomb´s balance

  • test charge - An increased charge causes the torsion wire to twist more.
  • torsion wire - The attracting and repulsive forces cause it to twist and induce torsional stress in it. When the torsional stress balances the torque, the balance reaches an equilibrium. Larger electrostatic force requires more twist in it for balance, therefore the magnitude of the force is proportional to the angle of the twist.
  • charged brass sphere

The torsion balance constructed by Charles Coulomb (1736–1806) is used to measure electrostatic interaction.

Electrostatic interaction causes the torsion wire to twist and induces torsional stress in it. When the torsional stress balances the torque resulting from the electrostatic interaction, the balance reaches an equilibrium. Larger electrostatic force requires more twist in the torsion wire for balance, therefore the magnitude of the force is proportional to the angle of the twist.

Cavendish balance

  • torsion wire - The gravitational force between the spheres causes it to twist and induces torsional stress in it. When the torsional stress balances the torque resulting from the gravitational force, the balance reaches an equilibrium. When the objects are heavier, a greater twist is required for balance. The magnitude of the force is proportional to the angle of the twist; if the mass of the objects is known the gravitational constant can be calculated.
  • telescope - The twist of the balance is observed through this.
  • looking in the telescope

The torsion balance constructed by Henry Cavendish (1731–1810) is used to measure gravitational force.

The gravitational force causes the torsion wire to twist and induces torsional stress in it. When the torsional stress balances the torque resulting from the gravitational force, the balance reaches an equilibrium. When the force is greater, a greater twist is required for balance. The magnitude of the force is proportional to the angle of the twist. The value of the gravitational constant can be determined with the help of Cavendish’s torsion balance: it is about 6.67 ∙10⁻¹¹ (N∙m²)/kg².

Eötvös balance

  • telescope - The twist of the balance and therefore the turn of the mirror is observed through this. When the mirror turns the image of the scale is also shifted.
  • scale - Its image is reflected by the mirror into the telescope. When the torsion wire twists the mirror turns and therefore the image of the scale seen through the telescope is shifted.
  • mirror - When the torsion wire twists, it turns and changes the angle of reflection of light. Therefore the image of the scale is shifted, which is observed through the telescope.
  • torsion wire - Due to the spatial variation, unevenness of the gravitational field different forces affect the two masses. The platinum-iridium torsion wire twists and torsional stress arises in it. When the torsional stress balances the torque resulting from the gravitational force, the balance reaches an equilibrium. The magnitude of the force is proportional to the angle of the twist.

The Eötvös balance is an improved, very sensitive variation of the earlier torsion balances, developed by Loránd Eötvös (1848–1919), a Hungarian physicist. As he wrote: ‘It is a Coulomb balance in a special form, that's all.’

The torsion wire is made of platinum, its twist, however small, can be observed with the help of a mirror. The rotation of the balance is caused by the changes of density of underlying rock strata, which is why it was widely used in oil and mineral exploration.

The Eötvös torsion balance is also suitable for determining the ratio of the heavy mass and inertial mass of a material. Loránd Eötvös’ precise experiments proved with high accuracy that these two masses are in fact the same. This result was the basic assumption of Albert Einstein’s general theory of relativity.

Animation

Narration

The torsion balance constructed by Charles Augustin de Coulomb is used to measure electrostatic interaction.

Electrostatic interaction causes the torsion wire to twist, which induces torsional stress in it. When the torsional stress balances the torque resulting from the electrostatic interaction, the torsion balance reaches an equilibrium. Larger electrostatic force requires more twist in the torsion wire to reach the equilibrium, therefore the magnitude of the force is proportional to the angle of the twist.

The torsion balance constructed by Henry Cavendish is used to measure gravitational force.

Gravitational force causes the torsion wire to twist and induces torsional stress in it. When the torsional stress balances the torque created by gravitational force, the balance reaches an equilibrium. When the force is greater, a greater twist is required for balance. The magnitude of the force is proportional to the angle of the twist. The value of the gravitational constant can be determined with Cavendish’s torsion balance, which is about 6.67 ∙10⁻¹¹ (N∙m²)/kg².

Developed by Hungarian physicist Lóránd Eötvös the Eötvös balance is an improved, highly sensitive variation of the earlier torsion balances. As he put it: ‘It is a Coulomb balance in a special form, that’s all.’

The torsion wire is made of platinum; its twist, even if it is very small, can be observed in a mirror attached to it. The rotation of the balance is caused by the changes in density of underlying rock strata, which is why it has been used widely in oil and mineral exploration.

The Eötvös torsion balance is also suitable for determining the ratio of the gravitational mass and inertial mass of various kinds of material. Loránd Eötvös’ precise experiments proved that these two types of mass are in fact the same. This result was the basic assumption underlying Albert Einstein’s general theory of relativity.

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