# TORSION BALANCE EOTVOS Eotvos built two different types of torsion balances for carrying out his gravitational investigations. The first type was a light horizontal beam suspended on a torsion wire with platinum masses attached to either end, so that the masses were at the same level (Curvature Variometer). This type was identical in form with the instrument used by Cavendish. The curvature variometer measures the «curvature» values which give the deviation of equipotential surfaces of gravity from spherical shape, and give the direction of the minimum curvature. The second type has a platinum mass attached to one end of the horizontal beam, while on the other end a platinum cylinder hangs on a wire so that the second mass is at a lower level than the other one (Horizontal Variometer).
The beam in both cases revolves around the torsion wire on a horizontal plane and is deflected from the torsionless position of the wire by the horizontal components of the gravity forces. This seemingly insignificant modification was Eötvös’ most important invention, in fact this second version is known as the Eötvös Torsion Balance.

The horizontal variometer gives the «gradient» of gravity which is defined as the rate of change of gravity over a horizontal distance of 1 centimetre. The horizontal variometer also furnishes the curvature values if the instrument is set up in the requisite number of azimuths. The gradients and curvature values are expressed in Eötvös units which are about 1*10^-12 part of the force of gravity.

In principle, the torsion balance is very simple. However, due to the extreme precision required it is a highly refined instrument. Its sensitivity can be illustrated by the following example. If a piece of metal weighing 1 gram were stretched out to encircle the Earth’s equator 25 times and 1 mm of the stretched wire were cut, the segment would weigh 1*10^-12 grams. This weight is in the order of the Magnitude of forces the Eötvös torsion balance could detect.

Eotovos’ instruments were realised in the Nándor Suess Precision Tool Workshop * predecessor of the Hungarian Optical Works (MOM). The Curvature Variometer and the Horizontal Variometer were completed in 1890 and tested in the lab and in the field in 1891. The simple gravity variometer completed in 1898 was a modified version of the horizontal variometer and was specially designed for field work. It was shown and awarded at the world exhibition in Paris in 1900.

In order to increase the efficiency of field work, Eötvös constructed a double instrument with two balances in antiparallel arrangement (1902). In later years the above-mentioned two instruments were used in experiments carried out by Eötvös and his associates to investigate the problem of proportionality of the inertial and gravitational masses.