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 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 the 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.
Following the German invasion, the developed prototypes are used for the detection and demining landmines laid down by the Dutch Army in Limburg and Zeeland.
After the development of the low-frequency model, work continued on the development of a high-frequency detector, even after the German invasion. The search coil is included in the ‘pendulum circuit’ of a high-frequency generator (+/- 275 kHz). There is a second, fixed high-frequency generator at 275 kHz that is connected to the first detector circuit. If the search coil comes close to a metal object, it destabilises the circuit’s frequency. The difference frequency then becomes audible. The design is such that a very stable base frequency is achieved and that the design is as little as possible temperature dependent. The system is based on three Philips D1F pentode tubes. A lot of attention is paid to the weight: 5.5 kg on the back and 4 kg for the search reel with handle.
The prototype device is ready in September 1940.
At the end of 1946, the Meetgebouw contacted the Demining School of the Army’s Demining Service (MOD) in Breda at the request of the Ministry of War. The MOD has about 800 pieces of the English landmine detector no. 3 in stock in Bergen op Zoom at that time. One is not very happy about the equipment. In January 1946, the high-frequency landmine detector developed by the Meetgebouw is tested and compared with the English no. 3 and no. 4 detectors.
The device of the Meetgebouw turns out to be much more sensitive. There are, however, some drawbacks: the device does not emit sound at rest, and, when metal is detected, it emits a low tone that may not exceed the noise of strong winds. The mechanical design is also not strong enough for the heavy fieldwork.
A new version is being developed, which also looks at the advantages of the English model no. 4 and the SCR 625 detector of the US Army. The interference tone is increased to 1000 Hz. A sharp elimination filter is also applied, whereby the detection of a piece of metal leads to a powerful signal. With the developed version, a flat-lying cent 1 to 2 cm from the search coil can be detected. That is far better than the English model no. 4 and the American model.
Acknowledgement and /reference
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.