Torsion spring
Other uses are in the large, coiled torsion springs used to counterbalance the weight of garage doors, and a similar system is used to assist in opening the trunk (boot) cover on some sedans.
Other more specific uses: 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.
[5] 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[6] 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.
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 two charged balls repelled one another, twisting the fiber through a certain angle, which could be read from a scale on the instrument.
Cavendish accomplished this by a method widely used since: measuring the resonant vibration period of the balance.
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 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.
, as is the case with torsion pendulums and balance wheels, the frequency of vibration is very near the natural resonant frequency of the system: Therefore, the period is represented by: The general solution in the case of no drive force (
), called the transient solution, is: where: The balance wheel of a mechanical watch is a harmonic oscillator whose resonant frequency
with weight screws set radially into the rim of the wheel, and then more finely by adjusting
In a torsion balance the drive torque is constant and equal to the unknown force to be measured
When the oscillatory motion of the balance dies out, the deflection will be proportional to the force: To determine
If the damping is low, this can be obtained by measuring the natural resonant frequency of the balance, since the moment of inertia of the balance can usually be calculated from its geometry, so: In measuring instruments, such as the D'Arsonval ammeter movement, it is often desired that the oscillatory motion die out quickly so the steady state result can be read off.
