1. DMI Development and World-wide Sale of Precision Magnetic Instruments High-Precision Magnetometers Measurement of the vertical geomagnetic component (middle) In the days of mechanical magnetometers the geomagnetic inclination was most often measured rather than directly measuring the vertical magnetic component. In Rude Skov Observatory an ”Induction Magnetometer” was used. The principle relies on the induction of an AC electric field in a coil rotating in the geomagnetic field. The induced voltage vanishes when the axis of rotation is parallel to the Earth’s magnetic field. Thus the minimum in induced voltage defines the direction of the geomagnetic field. QHM and BMZ Development and Production at DMI The QHM and BMZ magnetometer instruments constructed by la Cour and his assistants use modern magnet-steel (Alni also named Ørstit) for the permanent magnets and quarts-fibre for suspension of the magnets. This combination gave particularly stable instruments. The production and sale of magnetic instruments was strongly accelerated around the ”Second international Polar Year” (1932-33) and again during the ”International Geophysical Year” (1957-58) where many temporary geomagnetic observatories were established. The small white QHM (see photo) became Dan la Cours most famous and widely used instrument construction.. The young Dan la Cour. Photo from the DMI aurora expedition to Akureyri, Iceland, 1899-1900. The expedition was headed by Adam F. W. Paulsen, director of DMI. In 1920 Dan B. la Cour became head of the Magnetic Observatory in Rude Skov, a division of DMI, and from 1923 to 1942 he was director of the Danish Meteorological Institute. Masters of Geomagnetic Instruments From left, V. Laursen (assistant to la Cour), Johs. Egedal (head of the Geomagnetic Division), and Dan B. la Cour (director of DMI), at Rude Skov Observatory in 1935. DMI Magnetometers exported to Geomagnetic Observatories all over the world DMI holds a position among the worlds leading institutes for development and manufacturing of high-quality precision magnetometers for observatory use. This position was founded around the ”Second International Polar Year” (1932-33) by the brilliant work of Dan B. La Cour (1876-1942) assisted by Johs. Egedal and Viggo Laursen. They developed instruments like the ”Quartz Horizontal-force Magnetometer” (QHM) and the ”Balance Magnetometrique Zero” (BMZ) instruments. More recently a high-precision, ultra stable Flux-Gate Magnetometer (FGM) has been developed and produced by Emil Kring Lauridsen, Ole Rasmussen and Michel Genevey. The green regions in the world map indicate the countries where observatories have used the magnetometers exported from DMI. Theodolite Magnetometer mounted at the Geomagnetic Observatory in Qeqertarsuaq The instrument uses a method for abolute measurement of the horizontal component of the geomagnetic field invented by J. C. F. Gauss. The method has two steps. First an ”oscillation magnet” is mounted in different positions along a slide mounted at a declination variometer whereby its magnetic moment influences the suspended variometer magnet. Next the ”oscillation magnet” is mounted as a pendulum in a closed housing and its oscillation frequency, which relate to the horizontal magnetic component, is determined. From these measurements the absolute intensity of the horizontal magnetic field can be determined. The instrument was installed at the magnetic observatory in Qeqertarsuaq (Godhavn) in 1926 but has (since long) been replaced by modern magnetometers. Photo of Theodolite Magnetometer mounted at Rude Skov Observatory (right) la Cour QHM magnetometer Present DMI Magnetometer development and sale. The magnetometry work was later carried on by Emil Kring Lauridsen, Ole Rasmussen and Michel Genevey, who have developed and produced for sale modern flux-gate magnetometers. An ingenious cardan suspension mechanism for the tri-axial sensor elements mounted in a heavy marble block, careful selection of materials with negligible temperature variations, and precise and reliable electronic circuits have contributed to provide an instrument of superior quality. With a production of much over hundred flux- gate magnetometers the total sale of DMI high-precision magnetometers to Observatories all over the world exceeds one thousand instruments. Modern Observatory Flux-Gate Magnetometer Peter Stauning. Danish Meteorological Institute. September 2002. pst@dmi.dk World-wide sale of DMI magnetic instruments During the past 70 years since the Second International Polar Year 1932-33 the total sale of DMI high-precision magnetometers amounts to the following: Quarts Horisontal Magnetometers (QHM) of la Cour type: 798 units Quarts Horisontal Magnetometers (QHM) Kring Lauridsen type: 57 units Balance Magnetometer (BMZ) of la Cour type 343 units Fluxgate Magnetometer (3-axial)Kring-Genevey-Rasmussen type 145 units Thus the total sale of instruments to magnetic observatories and research institutes all over the world amounts to 1343 high-precision DMI magnetometers In addition to this number, hundreds of the less demanding H, D or Z variometers, and other instruments have been produced at DMI and sold around the world.