Sea and landmines: the period 1938 – 1941
Around 1938, the Laboratory for Physical Warfare (‘Measurement Building’) was approached by officers of the Royal Netherlands Navy about the problem of “antenna mines”. The spines of these anchored sea mines rub the skin of a ship blank upon contact. This caused a difference in voltage between these objects; the resulting electrical current caused the mine to explode. Although that problem was seriously studied, the Laboratory was unable to give a conclusive advice (based upon physics) about disabling or sweeping these mines.
End of 1939, or in the first months of 1940, Vice-Adm. Furstner told the Measurement Building about a new, probably magnetic type of sea mine that was used by the Germans in their war against the British Navy. He asked about how to approach this threat. After the Royal Netherlands Navy had caught such a sea mine and the magnetic ignition character had become clear, it was necessary to act quickly. The English Royal Navy was not willing to share information.
The assignment to the Measurement Building was to bring the near magnetic field of an ellipsoidal magnetic dipole into a differential equation and solve it. That assignment was set aside. However, work was performed on a plan to reduce the magnetic signature of ships by means of an electric current through loops of conductive material around ships and parts of ships. Tests which smaller naval vessels and a submarine proved that this approach was possible to a large extent.
In a next contact of the Royal Netherlands Navy with the British Royal Navy, the Dutch suggested their approach and success. Then the Brits stated that they were on the same track in developing countermeasures [PW]. As a matter of urgency, the Royal Netherlands Navy then ordered cables for current loops. The industry, however, could not deliver fast. Therefore, not so many ships were protected when war broke out between the Netherlands and Germany.
Probable early 1940, the Royal Netherlands Army asked the Measurement Building to develop a detector for iron landmines. When the war broke out in 1940, only a couple of landmine detectors were available of a model that passed the laboratory stage. That model, however, did not meet the mechanical requirements for robust military use. The principle of the detector was based on making the impedance change in a search coil (on a stick) when approaching iron objects and by making that change audible by means of a low-frequency bridge circuit. The conventional iron mines could be detected with certainty at a distance of 75 centimetres.
To a large part, this text is based on Prof. Van Soest’s contribution to the book ‘Physisch Laboratorium 1927 – 1977’.
[PW] Spycatcher, Peter Wright (1987) chapter 2.