I received these images of a device featuring a Junghans J35 movement from a fellow collector. Thanks to the guidance provided by Dr. Bernd Stiller of the "Museum für Meteorologie und Aerologie" we were able to navigate the path of further research, ultimately leading to a successful identification of the device.
About a Pocket Thermograph.
By W. Keil.
Notification from the Physikalisch-Technische Reichsanstalt (Physical-Technical Institute) 6. June 1935.
In the preparations of a procedure for testing high-performance pocket watches under the conditions of daily use, it appeared necessary to capture the temperature conditions for these operating conditions. Although watch technology has adopted high and low temperature values for compensation balances, which have been incorporated in better watches for some time, there is no information in the literature about the actual temperature conditions under which the pocket watch operates. However, this knowledge is as important for assessing the performance of a pocket watch as it is for understanding the factors influencing the timekeeping accuracy of the watch. The desire to comprehensively record the temperature variations led to the creation of the pocket thermograph.
Since no further uses were initially assumed for the small device, and considering the question of costs, it seemed practical to use components that are already employed in watch and instrument technology.
Fig. 1. Pocket Thermograph seen from above.
Figure 1 shows the device from the top view, Figure 2 depicts a cross-section. Adjacent to the dust-sealed pocket watch movement "u", which is wound via the extended winding stem "w" using the crown "i" and is fastened to the base plate "p", there lies the bimetallic strip "a". It is circularly bent, its length corresponds to a central angle of 240°, which yields the greatest possible adjustment range. It is fastened to the projection "v" of the holding piece "l" through screws. The movements of the free end of strip "a" caused by temperature changes are transmitted via the chronometer chain "b" to the axle "c", on which a lever "h" is fixed, extending over the upper axle bearing. The latter carries a light leaf spring "d".
The end of this spring carries a hollow cone "k" at its tip, which is equipped with a very fine bore to hold the writing ink, and it lightly rests on the writing disc "t" with spring pressure. The spiral spring "f" constantly maintains a slight tension on chain "b". In the pocket watch movement u, the so-called hour pipe makes only one revolution in 24 hours instead of two. The writing disc "t" with a diameter of 50 mm is fastened to the hour pipe and is secured in its position by the swiveling lever "e", which engages between a gear and the disc. Thus, the disc remains in continuous engagement with the movement "u". The downward protruding pin "m" on lever "e", when the writing disc is unlocked by swiveling lever "e", adjusts the bimetallic strip within its elastic limits so that the writing stylus is positioned outside the edge of the disc. This significantly facilitates the lifting of the recording disc. The pre-printed sheet with polar coordinates, gummed on the back, is moistened and then securely adheres to the disc. It can be easily removed from the disc without endangering the recording after exposing the disc with the writing sheet to steam from boiling water for a short time. The screw "g", whose eccentric head carries a square for attaching an adjusting key, can be used to adjust the position of the holding piece "l", allowing the 25°C measurement range of the device to be adjusted within certain limits.
Fig. 2. Cross-section through the Pocket Thermograph.
The screw "s", whose neck fits into the slot in holding piece "l", serves as guidance during the adjustment.
For the device, normal high-quality pocket watch movements were used, where only the translation of the hour pipe needed to be changed and the winding stem needed to be extended. The case used was one similar to those used by the company Lufft, Stuttgart, for pocket barometers.
Numerous such devices have proven themselves in longer observation series, which will be reported in another context.
Figure 3 shows a recording obtained with the device, in which significant temperature fluctuations during wear on the body are noticeable. The accuracy of the display reached the targeted accuracy of ±1°C, and it can be increased if line recording on wax paper is used.
Fig. 3. Recording of a Pocket Thermograph on the left, Redrawn in rectangular coordinates on the right.
The device can be used for purposes other than initially envisaged, as it requires minimal space, and its thermal inertia can be adapted to the specific purpose without difficulty.
In the development of the device's construction, I was supported by my brother, Mr. Dipl. Ing. G. Keil in Sarstedt, who prepared two construction drawings. I thank him at this point as well as the Deutsche Forschungsgemeinschaft (German Research Foundation) for providing funding, and Mr. Director General Helmuth Junghans in Schramberg for donating the movements.
On the right, you'll find an image showcasing the title page of the magazine where the aforementioned article was featured.
Below, you can observe a series of images portraying a successive iteration that was utilized to enhance the precision of temperature recording on the wrist. This advancement was aimed at facilitating in-depth research into wristwatch performance.
Original J35 movement copied in.
Altered J35 movement.
Cutouts to make room for the 24 hour minute wheel.
Altered Setting lever spring
Face locking and writing arm moving lever.
A Brief Overview of Wilhelm Keil:
Commemorating Wilhelm Keil's 70th Birthday
Born on February 3, 1891, in Herzogswald (District of Meißen), the young Wilhelm Keil attended the State Gymnasium in Dresden-N from 1903 to 1910. He then studied in Göttingen from 1910 to 1911 and in Munich from 1911 to 1914. Soon after the outbreak of the First World War, he was drafted into military service. In the spring of 1918, he managed to complete his doctoral work during a few weeks of leave.
Shortly after the war ended, his journey into the field of chronometry began. His first stop was the observatory in Hamburg-Bergedorf (1919 and 1920). He then worked at the German Hydrographic Institute until the end of 1923, followed by employment at the Askania Works in 1924 and the Aeronautical Observatory Lindenberg, branch Berlin-Tempelhof, in 1925. From October 1925 until his appointment at the Technical University of Stuttgart in 1944, he worked at PTR in Berlin.
In March 1930, a conference of the Metallurgical Research Committee of the then Emergency Association of German Science, led by Dr. Schmidt-Ott, took place in Stuttgart. During this conference, PTR Keil was sent as their representative, and discussions were held regarding collaboration between the watch industry and research in the industry. It was from this point that the planning began to establish a professorship for watch technology, horology, and precision mechanics at the Technical University of Stuttgart, along with a university institute. The professorship established in 1942 was offered to Keil in 1944.
Under challenging circumstances, Keil began the task of building up the institute. Nevertheless, through relentless efforts to secure space and resources, particularly for machinery and equipment, he succeeded in establishing the institute. Gradually, opportunities for work were provided to diploma and doctoral students.
After the establishment of a research society for watch and precision instrument technology, he, along with the watch industry, obtained the construction of an institute where he continues to serve as its director. This institute houses the professorship for watch technology, horology, and precision mechanics of the Technical University of Stuttgart, as well as the research institute for watch and precision instrument technology. While the former primarily serves teaching and research, the latter conducts purposeful and commissioned research.
In response to the watch industry, watch technology, and academia, the German Society for Chronometry was founded in 1949, with W. Keil elected as its first chairman. The society, of which he is still the head, has honored him on the occasion of his 70th birthday by electing him an honorary member.
G. Glaser, Schramberg, Physikalische Blätter - Februar 1961