3 - Schmidt's field balance
A Schmidt's field balance is an on-site variometer which allows measurement of field differences (H or Z) between two instants or between two points. These variometers are used in magnetic prospection to detect anomalies in the Earth field in the area under study.
Vertical component Z
|
| Schmidt's vertical balance, designed in 1914 in Berlin Meteorological Institute, was gradually improved until it became the most widespread prospection instrument between 1920 and 1950. Different versions were commercialised by the Askania and Ruska companies. The balance on view was used in 1932-1933 by J.P. Rothé in Scoresby Sund (Greenland) during the second International Polar Year. |
 | Schmidt's vertical balance is mainly made up of a long magnet moving around a horizontal axis. The axis consists of a steel knife resting on agates and a plane mirror attached to the upper part of the magnet. Thanks to the mirror, the knife's angular displacements can be observed using a vertical-axis, self-focussing glass. The whole of the magnet rotates around a vertical axis, with the position noted on a horizontal azimute circle. The design of the balance and its portable assemblage compensate in part for the temperature effect. The instrument's sensitivity can be largely adjusted by modifying the position of the floating magnet's centre of gravity. This is done using a screw situated on the lower part of the magnet. The instrument is controlled using two vertical axis Helmholtz reels. The resolution obtained is of about one nanotesla.
|
Horizontal component H
|
| The model on display was made by the Askania company before 1930. Over the years the company proposed several different improved models of the horizontal balance. Here the model on display has been opened to allow visualisation of the constituent parts.
|
Schmidt's horizontal balance functions on the same principle as the vertical one, except that its magnet is in a vertical position, and is therefore sensitive to variations in the horizontal component H of the Earth magnetic field. The floating magnet moves around a horizontal axis perpendicular to the magnetic meridian of the location.The quartz knives, which form the magnet's rotation axis rest on an agate plane. As for the vertical balance, the centre of gravity of the mobile assemblage can be modified using some screws judiciously placed on the balance beam.
