**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

**Related items**

### 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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