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Deutsches Technikmuseum - July

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The Websites of the donation Stiftung Deutsches Technikmuseum Berlin at a glance:

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Exhibit of the Month July 2017

Photo: A small, scaled glass tube containing mercury is encased in an elongated wooden frame. The small tube has a prominent bend at the upper end.

Deep-sea thermometer – an important invention for the exploration of a mysterious habitat © SDTB / Photo: C. Kirchner

Negretti & Zambra Deep-sea Thermometer, circa 1880

The measurement of deep-sea temperatures achieved a real breakthrough 160 years ago in 1857 when the first reliable deep-sea thermometer was invented.

Precise measurements of water temperatures in the deep-sea (over 200 meters) had previously not been possible because after a thermometer is dropped to deep depths it has to be pulled back through the upper water stratums which has different temperatures. The temperature shown by earlier thermometers was thus no longer accurate due to adulteration.

Precise temperature measurement by jolt

With the development of the deep-sea thermometer shown here, it became possible to fix the position of the temperature indicator at depth so it would not change when being pulled back up to the ship.

The constriction and the prominent bend of the glass tube containing the mercury are typical features of the British instrument producers Negretti & Zambra. When being allowed to sink to depth, the thermometer is aligned on the plumb line such that the glass tube´s empty collection reservoir points upwards.

Photo: The metallic company plaque. Inscription: NEGRETTI & ZAMBRA - PATENT STANDARD - DEEP-SEA THERMOMETER

Patented gauge developed in London; for entire photo please zoom! © SDTB / Photo: C. Kirchner

In order to set the temperature indicator when the desired depth is reached, the plumb line on which the wooden frame of the thermometer is attached must be yanked abruptly. The jolt causes the mercury thread to break at the point of constriction (breaking point).

The whole thermometer turns over such that the collection reservoir winds up at the bottom end of the instrument and the broken mercury thread falls into the reservoir. The thermometer is pulled back to the surface in this position. Once on board, the amount of the collected mercury can then be used to extrapolate the temperature of the corresponding stratum of the sea.

Such a deep-sea thermometer was used extensively on the research ship S.M.S. GAZELLE. In June 1874, this Prussian corvette set sail from Kiel on a two-year expedition to the Cape of Good Hope under the command of the future Vice Admiral Georg Freiherr von Schleinitz.