History of Geophysical Research in The Netherlands ... - DWC - KNAW

In the course of time geomagnetic recordings in the Sonnenborgh became more
and more disturbed. Especially perambulators, which were pushed through the
park, were a nuisance. At first they could be kept at a safe distance by means
of a thorn hedge, but near the end of the century a quieter place had to be
found. Therefore, geomagnetic recording was moved in 1898 from the Sonnenborgh
in Utrecht to the manor Koelenberg at De Bilt (in the grounds of the
present KNMI). A description of the geomagnetic instruments and buildings in
these new grounds can be found in the publication "Mededelingen en Verhandelingen
van het KNMI" lA, and also in Yearbook-B, 1915.
Only for a decade the recordings were there free from artificial disturbances .
Then electrification of the former horse-drawn tramway, Utrecht-De Bilt-Zeist,
which was completed in 1909, caused problems. The tramway used direct current;
as a consequence magnetic disturbances were caused by the currents
flowing through the overhead wire and the rails, as weIl as by stray currents
that expanded from the rails over great distances and disturbed the natural
geomagnetic field. A great but temporary improvement was achieved by the
use of a second overhead wire (in 1911), which carried the return current instead
of the rails.
However, the situation again worsened. In 1938 the greater part of the network
of the Netherlands Railways was electrified. Direct current was used,
just as for the tramway, but of much greater strength . The only solution was
to move the geomagnetic observatory. A suitable site was found in the hamiet
of Witteveen (between Westerbork, Emmen and Hoogeveen in the province of
Drenthe). In the middle of 1938 a new observatory was put into use. A description
of the instruments with pictures of the buildings is given in KNMI-Yearbook
Geomagnetism, 1939. Until 1952 recordings of the geomagnetic variations
in the Magnetic Observatory at Witteveen were ideal. In May 1952 the Zwolle­
Groningen railway was electrified and this ended the period at Witteveen in
which no artificial disturbances occurred. Since 1952 instruments in the Magnetic
Observatory at Witteveen can no longer record in the finest details the
natural variations of the geomagnetic field.
The results of the recordings made by the KNMI have been published since
1903. From 1903 to 1938 the hourly values of the geomagnetic components were
derived from the records at De Bilt, and since 1938 the hourly means of the
components recorded at Witteveen . They have been published in the Yearbooks
of the KNMI (KNMI, 1903 to the present).
The possibility of carrying out field measurements with improved instruments
made a repetition of the magnetic survey of 1891 attractive. In the years 1942
to 1948 the geomagnetic field of the Netherlands was measured at about 400
places. The results have been published by J. Veldkamp in a KNMI publication
(Veldkamp , 1951). The anomalies of the geomagnetic field are related to the
subsurface structure in this country. This fact was established also by Ph.e.p.
Hartmann in his doctor's thesis (Hm'tmann. 1945), which was based on the geomagnetic
survey of 1942-1948, and on the geological knowledge at that time.
J.A. As analyzed secular variations (the slow variations that occurred in the
geomagnetic field during the last decades) and drew conciusions with respect
to future variations (As, 1967). The secular variations appear to be strongest
in some beits around the world that coincide more or less with the global tectonic
belts. J. A. As surmised that in these belts processes in the earth 's core
are linked with those in the mantIe of the earth (As, 1975).
In the period from 1960 to the present modern instruments have been devised
and constructed at the KNMI for measuring the earth 's magnetic field and its
components, such as proton magnetometers (also for use at sea) and a proton
vectormagnetometer as an observatory instrument. J.A. As also developed in-
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