page_title: Torsion spring
section_title: Torsion balance
hierarchical_section_title: Torsion spring / Torsion balance
caption_attribution_description: Русский: Крутильные весы КулонаEspañol: Balanza de torsión de CoulombItaliano: Bilancia di torsione di CoulombNederlands: Torsiebalans van Coulomb met verschillende componenten zoals Coulomb die omstreeks 1777 zelf schetste in één van zijn wetenschappelijke publicaties.
context_page_description: A torsion spring is a spring that works by twisting its end along its axis; that is, a flexible elastic object that stores mechanical energy when it is twisted. When it is twisted, it exerts a torque in the opposite direction, proportional to the amount it is twisted. There are various types:
  A torsion bar is a straight bar of metal or rubber that is subjected to twisting about its axis by torque applied at its ends.
  A more delicate form used in sensitive instruments, called a torsion fiber consists of a fiber of silk, glass, or quartz under tension, that is twisted about its axis.
  A helical torsion spring, is a metal rod or wire in the shape of a helix that is subjected to twisting about the axis of the coil by sideways forces applied to its ends, twisting the coil tighter.
  Clocks use a spiral wound torsion spring sometimes called a "clock spring" or colloquially called a mainspring. Those types of torsion springs are also used for attic stairs, clutches, and other devices that need near constant torque for large angles or even multiple revolutions.
context_section_description: The torsion balance, also called torsion pendulum, is a scientific apparatus for measuring very weak forces, usually credited to Charles-Augustin de Coulomb, who invented it in 1777, but independently invented by John Michell sometime before 1783.  Its most well-known uses were by Coulomb to measure the electrostatic force between charges to establish Coulomb's Law, and by Henry Cavendish in 1798 in the Cavendish experiment to measure the gravitational force between two masses to calculate the density of the Earth, leading later to a value for the gravitational constant.
  The torsion balance consists of a bar suspended from its middle by a thin fiber.  The fiber acts as a very weak torsion spring.  If an unknown force is applied at right angles to the ends of the bar, the bar will rotate, twisting the fiber, until it reaches an equilibrium where the twisting force or torque of the fiber balances the applied force.  Then the magnitude of the force is proportional to the angle of the bar.  The sensitivity of the instrument comes from the weak spring constant of the fiber, so a very weak force causes a large rotation of the bar.
  In Coulomb's experiment, the torsion balance was an insulating rod with a metal-coated ball attached to one end, suspended by a silk thread.  The ball was charged with a known charge of static electricity, and a second charged ball of the same polarity was brought near it.  The two charged balls repelled one another, twisting the fiber through a certain angle, which could be read from a scale on the instrument.  By knowing how much force it took to twist the fiber through a given angle, Coulomb was able to calculate the force between the balls.  Determining the force for different charges and different separations between the balls, he showed that it followed an inverse-square proportionality law, now known as Coulomb's law.
  To measure the unknown force, the spring constant of the torsion fiber must first be known.  This is difficult to measure directly because of the smallness of the force.  Cavendish accomplished this by a method widely used since: measuring the resonant vibration period of the balance.  If the free balance is twisted and released, it will oscillate slowly clockwise and counterclockwise as a harmonic oscillator, at a frequency that depends on the moment of inertia of the beam and the elasticity of the fiber.  Since the inertia of the beam can be found from its mass, the spring constant can be calculated.
  Coulomb first developed the theory of torsion fibers and the torsion balance in his 1785 memoir, Recherches theoriques et experimentales sur la force de torsion et sur l'elasticite des fils de metal &c.  This led to its use in other scientific instruments, such as galvanometers, and the Nichols radiometer which measured the radiation pressure of light.  In the early 1900s gravitational torsion balances were used in petroleum prospecting.  Today torsion balances are still used in physics experiments.  In 1987, gravity researcher A.H. Cook wrote:
  The most important advance in experiments on gravitation and other delicate measurements was the introduction of the torsion balance by Michell and its use by Cavendish.  It has been the basis of all the most significant experiments on gravitation ever since.
caption: Drawing of Coulomb's torsion balance. From Plate 13 of his 1785 memoir.
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