History: The Physics Laboratory during WW II – the work


History: The Physics Laboratory during the German Occupation

Former director Prof Dr.Ir Van Soest wrote his memories about the German occupation period for the memorial book “Physisch Laboratorium TNO 1927-1977”. That contribution is reproduced almost in full below. 

Early 1940 

A few months before the outbreak of war in the Netherlands, Lt.Col. van den Bergh found a well-known German officer on his doorstep, who nervously informed him that “scary” things were coming for the Netherlands, that the Dutch had to be ready and had to hit back as hard as possible. It was an extremely short visit from an apparently anti-Hitlerian. At the Department, the passed-on message was – according to Van der Bergh – taken lukewarm. For us, it was certain that disaster was coming. On May 10, 1940, high military authorities were scheduled to watch the operation of “I.F.F.” which was installed in the tower of the St. Jacobs Church. The “electric listening device” was installed on the roof of the Meetgebouw in Waalsdorp. It turned out differently. 

The German invasion 10-14 May 1940

It started at about half past six on May 10, as far as I remember. At 07:00 I was in Waalsdorp, having passed a bombed-out shed of the Alexander barracks with holes in the stone walls, a dead or wounded horse, and a machine gun fire path. The damage to the Meetgebouw was limited to broken windows caused by bombs that fell nearby and were aimed at the Waalsdorp camp. 
My employees arrived partly earlier, partly later than usual. A start was immediately made to secure the electric listening device by taking it down from the roof. A pit was dug to house the earlier captured, mostly German, radiosondes after being crushed. By telephone, Lt. Col. van den Bergh ordered to make the Waalsdorpervlakte (at the time a really flat terrain) unusable for plane landings. Rumour was that German paratroopers had landed in the Wassenaar dunes. Therefore, the secret archive of the Measurement Building had to be moved to safety. It was moved to my house and left under my bed. The secret equipment, if not in direct use, was collected in a wooden barrack, together with a few cans of gasoline to be set on fire in an emergency.

In these war days, hard work was still being done to get those things ready which could still be useful for the defence. An electric listening device was set up in the Hertenkamp (Koekamp) next to a machine gun. [a source states that only on May 14, troops with a machine gun took position near the Koekamp]
Another electric listening device was transported (by ir. Piket and helpers) through a German parachutist-occupied area to IJmuiden for shipment to England.

Model of the "electric listening device" (Dutch radar)
Model of the “electric listening device” (Dutch radar)

Of the experts in the field of the ‘electrical listening device’ ( “radar”), two were selected to cross over to England (von Weiler and Staal). This happened at the last minute; von Weiler left the port of Scheveningen after he had received the documents about this aircraft from me.
Ir Gratama examined whether an embassy maintained secret radio communications. And so on.

On the day of surrender by The Netherlands, I was the last person to walk down the (Oude) Waalsdorperweg after work, still unfamiliar with what had happened. There I met Commander van den Bergh, who had already met the seconded officer van Dijl (a physics MSc). I heard the message from the former. We went back to Waalsdorp, set fire to the wooden barracks with the secret devices and walked to the camp of the Grenadiers and Hunters. There was disorder. Officers were not there at the time, some non-commissioned officers tried to create some order; among the soldiers, some went about crying. Superior van den Bergh called everyone together in the courtyard and addressed them. I ended with a “Long live the Queen“, which was generally repeated. A voice behind me: “The Measurement Building is on fire“.

May 11 – mid-June 1940

A German regiment settled in Camp Waalsdorp. When I wanted to reach the Measurement building on May 18, I was stopped. Cramer, who arrived with me, I sent back to stop the others. Under the guidance of an officer, I had to point where I wanted to go. I was told everything was closed until further notice. Nicola, the reserve lieutenant of the Military Weather Service, also joined and returned. Once more I came with superior v.d. Bergh returned, who reported that there was private property (binoculars) of his in the Meetgebouw. In addition to these, tools were also stolen. A complaint to the German commander was dismissed with the statement that “Zivilists” must have done that.

During the closure, the staff of the Measurement Building met in the Inspectorate of the Artillery at the Jan van Nassaustraat. We gave lectures to each other and philosophised about how the work could be continued, for example, to improve the physical, electronic and mechanical capacity within the laboratory for civilian purposes. We decided, if at all possible, to stay together. The stoves were still burning to laboriously destroy the many archive documents, both at the Inspectorate and at my house (and presumably elsewhere).
Colleague Insje had previously photographed in miniature the entire archive with drawings. The photos were deposited in an empty matchbox and kept by me. [A second copy exists; see attached photo]

Micro photos were made of the drawings from the archived drawings before the originals were burned. They were hidden in a matchbox. 
Micro photos were made of the drawings from the archived drawings before the originals were burned in early May 1940. They were hidden in a matchbox.  

It didn’t take long – I estimate in the first half of June – before the Germans made some movement. I was taken to Waalsdorp and questioned by an officer who had come over from Germany about what was happening in this laboratory. Apparently, the German side was not aware of us at all. The interrogation was not difficult. I was able to talk extensively about the acoustic listening device: it was already outdated anyway. I could also be detailed about the meteorograph; this was already known from the literature. Since I had meanwhile learned from the Genie that the portable ultra-short wave transceiver had been captured by the Germans from the troop, I could also speak about this. I was asked if I believed that aircraft could also be detected by radio waves. I replied – as if a light had come on – that perhaps this would not be impossible. My answer was silly enough not to elaborate on it. Other matters – such as measures against magnetic mines – were not discussed. To his surprise, I refused to be taken back home by a German car and walked back.

Mid-June 1940 – 1 July 1941

Shortly afterwards we were allowed to re-enter the Measurement Building and start with an assignment from the “Settlement Office of the Department of Defence”. In the Secret letter no. 128 from the Committee for Physical Armorment of 17-06-1940 to the department and a quick reply from the Fifth department, no. 6 of 18-06- 1940, four assignments were given to the laboratory. This was stated in Secret letter no. 132 written by the secretary to the department on 05-07-1940, for application for funds. The committee chairman also wrote a letter to the department on 12-07-1940.
Het Meetgebouw has not been able to find out what these four assignments were, except for one: undoubtedly the most important. This concerned the further development of landmine detection devices. In 1940, the Dutch military had to clear the minefields laid in the Netherlands; they had nothing but “prods”. The order was accepted because – in our opinion – this clearing would not damage the Allied act of war and was based upon the work that already started before the German invasion. 

Probable early 1940, the Royal Netherlands Army asked the Measurement Building to develop a detector for iron landmines. When the war broke out in May 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 making that change audible using 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 were used for the detection and demining of 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 in 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 the handle.
The prototype device was ready in September 1940.

Schematics of the HF-landmine detector
Schematics of the HF-landmine detector


High Frequency (HF) landmine detector: handheld detector (amplifier and coil); battery and electronics as backpack unit
Landmine detector: a high-frequency handheld detector (amplifier and coil); battery and ‘electronics’ as backpack unit


Land mine detection - backpack unit and handheld detector
Landmine detection – backpack unit and handheld detector


Land mine detection - handheld detector
HF landmine detection – handheld detector

The technician A.J. Hendriksen carried out the first searches and also supervised afterwards.

[Note] In the period 1940-1941, work was also continued on several earlier developments, such as the development of an infrared roadblock following the infrared river barrier. The archive contains the electronic design (A) of the (receiving) amplifier and army green(!) physical cabinets (October 19, 1940) and an improved design (B) from March 1941. The width of the infrared ‘alarm gate’ was 50 to 100 meters. Philips D1F tubes (“Wehrmacht” pentode) were used.

In July and the following months, up to 1941, I began to collect a large portfolio of orders in the civil sector. Visits were made to many institutes in The Hague, Delft, Leiden, Lisse, Aalsmeer, Amsterdam, etc. Almost everything was aimed at the development of devices, often of an electronic nature, for the benefit of physicians and biologists; but also fine mechanical construction work was there. Assignments came easily, not only because of the many accidental connections I had in various fields, not only because people were remarkably willing to help, but also because the assignments for the clients in many cases involved little or no financial consequences. We were able to do something for the funds still requested by the committee and a supplement that had been received from the Settlement Office.

Our archive contains a surprising letter from Van Soest who, on July 5, 1940, two months after the German invasion, submitted a secret budget request to the Ministry for compensation of NLG 9,000 in resources lost as a result of actions by the German occupiers.

Not all contacts led to a direct “order” or “assignment”, but about forty are mentioned in the papers.

  1. High-frequency hardening of razor steel (Artillerie Inrichtingen, Rijswijk). A device was developed to harden steel strapping on both edges at high frequency. Such steel was needed for the manufacture of razor blades and tools.
    Auxiliary device for hardening steel strap with a tensioned steel strap (1941)
    Auxiliary device for hardening steel strap with a tensioned steel strap (1941)

    Device for hardening steel strapping (1941)
    Device for hardening steel strapping (1941)
  2. High-frequency diathermy device (Department of Rheumatic Disorders and Physical Therapy, Academic Hospital, Leiden: Dr. Goslings) [1]
  3. Study of the dielectric constant variations of the human body (Psychiatric Clinic, Oegstgeest, Leiden: Prof. Carp).
  4. Recording devices for psychogalvanic reflex, electrical plethysmograph (ditto).
  5. A “heatless” light source (Plantenphysiologisch Laboratorium G.U. Amsterdam).
  6. Electrocardiograph with a special time base (Department of Medicine, Leiden University Hospital: Dr. Snellen). Commissioned 4 December 1940; delivery report on 29-05-1941 [Piket].
  7. Tissue tearing device (Government Purchasing Office).
  8. Torsional tension meter (ditto).
  9. DC amplifier that was refined for physiological purposes. The aim was to measure the action currents of eye muscles and nerves (Wilhelmina Gasthuis Amsterdam).
  10. Vibration sensor as a pulse indicator for use with the electrocardiograph of the (Academic Hospital in Leiden).
    Vibration sensor (1941)
    Vibration sensor (1941)

    Pulsation frequency of the heart transducer (1941)
    Pulsation frequency of the heart transducer (1941)
  11. Torsion meter (Department of Mechanical Engineering, TH Delft).
  12. Equipment for electrical insect control (Plant Protection Service, Aalsmeer; Laboratory for Horticulture-Plant Cultivation, Wageningen; Laboratory for Flower Bulb Research, Lisse).
  13. Two millivolt meters (Rijksinkoopbureau).
  14. Electric herbarium disinfection (Rijksherbarium, Leiden). ‘
  15. Making several cardiograms simultaneously on an electro-cardiograph including a photographic image (Department of Medicine, Academic Hospital Leiden: Dr. Snellen) and one for Dr. J.B. Kleyn, The Hague (post-war article).&nbsp ;
  16. Metabolic meter (Pharmacological Laboratory of the Leiden University).
  17. Thermoelectric skin thermometer (Academic Hospital, Leiden). Note: delivered in 1941; in April 1942 returned for repairs.
  18. Microphone to measure the sensitivity of the human ear (Wilhelmina Gasthuis Amsterdam).
  19. Microphone for measuring heart sounds (delivery in April 1942).

When I came to Leiden at the end of the war period to take another look at the device for herbarium disinfection (a “Dender” with a large transmitter tube, supplying energy between two large condenser plates), some ordinary radio receivers had been attached to it outside, in the hope to save these aircraft from compulsory surrender to the Germans!

Research was also carried out into:

  • An infrared amplifier [1941, folder 8].
  • Enabling two simultaneous recordings with one cathode ray oscillograph.
  • Investigation of a cathode ray oscillograph with four timelines using an electronic switch.
  • A first study of measuring changes in conductivity and dielectric constant of living tissues.
  • Measuring local variations in the magnetic field. Another report has been preserved, but the background of why this aircraft was built is unknown.
    How it worked: Two equal coils with many turns were rotated on the ends of long shafts. In a homogeneous magnetic field, the electric alternating voltage thus generated is equal in magnitude and phase. When the coils are switched on against each other, the voltages cancel each other out. In a non-homogeneous field, the remaining residual signal is a measure of the inhomogeneity. The coils are located in the round non-magnetically conductive boxes at the ends of the shafts. A very accurate measurement was obtained by compensating the difference field with a homogeneous magnetic field generated by Helmholtz coils placed around one of the coils of the system. The field of the Helmholtz coils is then equal to the field deviation.

    Measuring device for measuring weak differences in the magnetic field (1941 - 1942)
    Measuring device for measuring weak differences in the magnetic field (1941-1942)

With the help of the forty orders, we managed to convince the Germans of the importance of maintaining the laboratory. The personnel continued to receive wages from the Settlement Office of the Department of Defense. As best I could, I kept in touch with the chairman and secretary of the former Commission. A German officer, apparently designated for control, occasionally came to see what we did. He was shocked when we were working with radio transmitters (diathermy device, herbarium disinfection, etc.). There was then a ban (not unjustly on his part!). The ban was not followed. I had visited a Philips employee of the Physics Laboratory known to me to obtain a number of these transmitter tubes. “Good heavens, what are you going to do with them?” “What do you care?” I got them. Part of them went into the underground transmitters that we made that were not part of the forty order numbers. The German officer in question must have been a decent man; however, he was transferred and apparently, he did not dare to leave the laboratory liberally and hand it over to a successor.

From July 1, 1941, to October 1943

After a short preparation, on June 28, 1941, a letter from the “Deutsche Kommissar für die Demobilmachung der Niederländische Wehrmacht” about “Übergabe des Physikalischen Laboratoriums an die Niederländische P.T.T.”, was followed by a letter from the Head of the Settlement Office to the Director General of the State Company Post, Telegraphy and Telephony (P.T.T.) of July 1, 1941, section E, no. 78. There was little choice. In the event of non-follow-up, we would be “vernichtet” as was verbally announced. The P.T.T., then still in good hands, cooperated. After an effective investigation by ir. Boetje and dr.ir. Koomans, the laboratory became “Physisch Laboratory P.T.T.”. I had previously made attempts to convert the laboratory into a “State High Frequency Service”, but no support was forthcoming. TNO did not dare to take over the laboratory either. As Physics Laboratory PTT we were able to continue the accepted assignments. As an institute, we had a somewhat less uncertain background than before.

A battalion “Grüne Polizei” settled in Waalsdorp, and seized the building of the former Military Weather Service, which was “taken” by us, but as military property, it was known to the Germans. From time to time other attempts by the German side to take the Physics Laboratory were also apparent; I can no longer reconstruct the case that prompted me to complain to the Grüne Polizei. In any case, I turned to the commander to be safe from intrusion. Smoother than I thought a sentry came to the door all the time the laboratory was still in Waalsdorp: one or two men more from the fighting strength! The sentries never understood why they were there, but “Befehl ist Befehl”.

Underground equipment left Waalsdorp under their eyes, often led by one of our youngest, and most silent: W. Cramer. But times got angrier. The P.T.T. came under N.S.B.’s leadership. An Einsatzleiter appeared in the Measurement Building, a vicious, dim-witted man. The organisation became such that he could only approach me through a few intermediaries in the rapidly established organisational context so that I was usually informed in advance of his plans. The tense contact broke down one day after he had accused me of being absent one day (I was attending a scientific meeting). I was chased away (early summer 1943) and with me, some of the academics.

The P.T.T. agreed with my proposal to hand over the management of the laboratory to ir S. Gratama, who continued this arduous task until the end of the war. He managed to save the laboratory from any work that could have been of any use to our enemy. I moved to the Radio Laboratory P.T.T..


Evacuation order from the German occupiers at the end of 1943 (source: Beeldbank van het NIOD)
Evacuation order from the German occupiers at the end of 1943 (source: Beeldbank van het NIOD)

The laboratory had to be evacuated in September/October 1943, because the Benoordenhout became “the stronghold of Seys-Inquart”. The lab was housed at the P.T.T. Central Warehouse Work and continued there for biological and medical applications. Hardly any serious work was performed for the benefit of the German occupier.  The most annoying was an order for sheet metal work for a German P.T.T. transmitter. The insulation boards were marked with a heavy (conductive) pencil before cutting; according to reports, this transmitter never worked due to the many carbon breakdowns that resulted from this signing. A great deal of time was spent on the production of the lectern (up to and including a wine cooler!) for a “high” German postal official, following the example of an even higher official! Meanwhile, the underground work also continued.

Underground work (1942 – 1945)

A very extensive underground work was the construction of a large broadcasting transmitter for use during the liberation, on which everyone counted. Under the leadership of Ir. Gratama, Insje was building this “high-frequency heating device”; the construction of the antenna was postponed. Few were aware of this work: outside the laboratory only the director of the Central Workshop and me. The underground work of communication transmitters and receivers ran across Capt. Boers van de Genie and when he went into captivity, about his wife. The first transmitter was temporarily deposited in the basement of the Inspectorate Engineers on the Raamweg, of which the chief, Col. Scharroo was ignorant. One day later the house of the Inspectorate was sealed by the Germans for further investigation. That day, Capt. Boers warned me. That same night, he lugged the transmitter with a Department of Defence porter through the gardens and the back of the neighbour’s house to safety!

The following page contains a personal report on the period from mid-1941 to 1945 by laboratory assistant Albert (Ab) Zuurmond.




[1] Zie ook: moderne apparatuur in de gezondheidszorg in: 5 Nieuwe structuren in de gezondheidszorg 1920-1940