Meteorograph (1934 – 1940)
Atmospheric data are of great military importance for, for example, aircraft missions and artillery. Because the Military Weather Forecast Service shared the Measurement Building with the researchers, this may have been the cause of an assignment to the researchers in 1934. The assignment was to develop the lightest possible construction for under a weather balloon, which could transmit Morse encoded data of three meteorological instruments, a so-called a meteorograph. A meteorograph is an instrument for automatically recording various meteorological conditions at the same time, in this case, barometric pressure, humidity and temperature. An earlier meteorograph was already used by the Russian researcher Moltchanoff in 1928.
Comparison with more than ten foreign designs of meteorographs from that time shows that the final model developed by the Measurement Building was an ingenious combination of a solid mechanically controlled scanning system with corresponding Morse coding for three electronic transmitter signals. The vertical rise of the balloon was initially used to set a propeller that would drive the coding mechanism. This drive proved to be unreliable, partly due to the high extra weight of the propeller. The propeller was therefore replaced by an electric motor.
Constantly revising the requirements and making them suitable for civil use delayed the development. The meteorograph hung between tensioned wires. The wires directly under the weather balloon acted as an antenna for the electronic transmitter. The battery for the electric motor and the transmitter hung on the wires under the meteorograph. The battery was protected from low temperatures by a cork wrapping. That wrapping also protected the device against the impact when returning to earth. Moreover, the meteorograph had a corrugated metal casing to protect the instruments against rain and direct solar radiation.
Between the end of 1939 and the capitulation of the Netherlands in May 1940 approximately one hundred meteorographs were manufactured industrially. A mass production of 42 to 45 meteorographs per week in wartime was prepared. Molds and stamp tools were developed. There are still several meteorographs on display in the museum.
Foreign weather probes landed on Dutch soil with some regularity. From the outbreak the first armed conflicts in Europe in 1939 on, these probes were no longer returned but stored at Waalsdorp. In 1940, at the time of the German invasion, there were about fifty probes in storage including many German probes. Those probes were immediately destroyed in a pit dug on the plain of Waalsdorp near the Measurement Building.
Technical details of the Meteorograph
The meteorograph records the temperature using a curved bi-metal thermometer, the humidity with a hair hygrometer, and the air pressure with a Bourdon type barometer (an air-tight curved barrel that stretches or rolls in with changing air pressure). Each of these three meters moves a sensor via levers over a curved holder wound in parallel with two (barometer), three (hygrometer) and four (thermometer) wires.
One of the wires of these holders uses a connecting roller for the electric connection with a cylindrical switching roller. This cylinder, which is driven by an electric motor via a worm gear rotates with one revolution per second. The drum of the cylinder is made of insulating material and is covered on the outside with a conductive pattern for Morse coding. Contacts have been placed on this cylinder, the so-called givers.
For the barometer where the course of the air pressure during the rise of the weather balloon with approximately 5 metres per second is almost exponential, the measurement moments for this air pressure have been chosen in accordance with that exponential trajectory. Only after the 4th, then after 15, 32, 63 and so on polling moments, the barometric pressure indication is transmitted via contact 8 of the cylindrical roller. The hygrometer measurements follow the possible varying values (e.g. showers) of the humidity in precise steps. The hygrometer is subsequently connected to the sliding contacts 6, 5 and 7 of the cylindrical roller. Because the temperature can vary greatly in the different air layers that the weather balloon passes, it has to be recorded as accurately as possible. As indicated, the switch roller rotates at one revolution per second. For each revolution, two Morse coded connections are subsequently made with the transmitter.
The hygrometer, in combination with the barometer, preceded by that of the thermometer, gives the following Morse code:
|Morse||Switching roller connection||Morse||Switching roller connection|
|D||_ . .||8 and 5||S||. . .||9 and 5|
|K||_ . _||8 and 6||U||. . _||9 and 6|
|G||_ _ .||8 and 7||R||. _ .||9 and 7|
Morse codes for transmitting the temperature:
|Morse||Switching roller connection|
In order to be able to distinguish the course in temperature, both ascending and decreasing, the following order of Morse signs for continuously rising temperature is included in the contact job: o, i, a, n, i, a, n, o, a, n i, a, n, i, o, n, i, a, n, i, a, o, i, a, n, and so on according to the following switch rolling figure. So a sequence of i, a, n where in each seventh coding the i, a, or n is been replaced by an o (4 – line in figure below).
Transmission of the code to the transmitter is realised by a vibrating transducer which turns the battery voltage of the single triode transmitter tube on and off in the rhythm of the Morse code transmission. The primary winding of the vibrating transducer is switched to earth in that rhythm. The frequency of 600 Hz of the vibrating transducer is fed to the grid of this triode. Using a transmitting frequency of 50 MHz (wavelength is 6 meters), the Morse code is transmitted amplitude modulated at 600 Hz.
The thermometer and barometer, after being mounted in the meteorograph, were calibrated in an evacuated and cooled vessel. The thermometer sensor was positioned in such a way that it could be controlled both forward and backward by the bi-metal thermometer in order to cover the entire expected temperature range.
The hygrometer was tested separately. The total weight of the meteorograph with a 4.5 V flashlight battery was 465 grams which allowed ascents up to five kilometres. For measurements at even higher heights, two batteries were used, one for the anode and filament feeding and the other for the electric motor. The extra battery increased the total weight of the meteorograph to 585 grams.
Signal reception on the ground took place with a simple super-regenerative receiver. Measurement distances up to 100 kilometres were possible. A special directional antenna was also developed to determine the map angle (azimuth) of the broadcasting weather balloon. The measurement data were recorded and processed by the Military Weather Service. After reaching the stratosphere, the balloon cracked. The radio device would come down connected to a parachute. A label attached to the probe asked the finder to hand over the remnants to the laboratory against a reward of fl. 7 1/2.-, a high amount for that time. After the return of the meteorograph on earth, the finder of the instrument was able to return it to the Military Weather Service.
A month before the outbreak of the Second World War, an article by Prof. dr. J.L. van Soest about the radio probe was published in the Tijdschrift van het Nederlandsch Radiogenootschap. A copy can be found here(pdf) [in Dutch].