2 - Q.H.M. 1

Designed in Denmark in 1934 by D. La Cour and commercialised in different versions until 1975 by the magnetic service of the Danish meteorological Institute, this remarkably simple and trustworthy instrument was used extensively in observatories for over fifty years.

A stirrup hangs from a solidly fixed quartz thread. A small horizontal magnet and a plane mirror perpendicular to its magnetic axis hang from the stirrups. This system is placed in a copper tube with two windows; through one of them, the magnet and its blocking system can be observed; the other has a self-focussing glass which can detect rotations of the magnetic equipment. The quartz thread is twisted a complete number of half-turns. The magnet thus moves from the magnetic meridian by a measureable angle, the sine of which is inversely proportional to the intensity of the horizontal component H of the Earth magnetic field.

The Q.H.M. is a relative measurement instrument; it must be calibrated in an observatory where the H component is known. The instrumental constants determined during the instrument's initial calibration are extremely stable; however, to allow for accidental variations, it is recommended to use three instruments during measurement campaigns, in case of possible instrumental faults. The precision in the H value is often up to one nanotesla for a correctly calibrated instrument.