2. Determination of the Deep-Sea Temperature.

All temperature-readings have been referred to the indications of the Hydrogen-Thermometer.
For the determination of deep-sea temperature we adopted two different methods, namely, with the Insulated Water-Bottle and with the Reversing Thermometer, and these two methods we tried to develope to their highest degree of perfection, hoping thus to be able to determine the temperature in situ with at least an accuracy of ± 0.01° C. We regret to say that the first attempt was to some extent a failure, as a great part of the deep-sea temperatures determined during the first cruise (1900) can hardly claim such a degree of accuracy, the reasons being that our Reversing Thermometers were not sufficiently well made, and that the Insulated Water-Bottle, in which we placed most confidence at that time, is incapable of giving accurate temperature determinations for great depths, owing to the disturbing effect upon the indications of the thermometer produced by the dilatation of the solid parts of the water-bottle (especially the ebonite, the celluloid, and the india-rubber) when being hauled up, this being subsequently proved by experiments. During the latest cruises of the Michael Sars , however, observations of the temperature of the deep strata of the Norwegian Sea have been made at several Stations with the new Richter Reversing Thermometers, and the temperature-determinations thus obtained have an accuracy of ± 0.01° C.

( a ). The Reversing Thermometers

Reversing Thermometers were used during the cruise of 1900 and in 1903 and 1904. In 1900 we had some long Reversing Thermometers specially constructed by Nansen, and made by NEGRETTI & ZAMBRA, and also some small NEGRETTI & ZAMBRA Reversing Thermometers, of which only one, a very trustworthy thermometer, was used during the cruise. In 1903 and 1904 Reversing Thermometers from C. RICHTER, Berlin, were used.
The chief difficulties with the Reversing Thermometers had hitherto been,

1. That the mercury did not always break off exactly at the same place in the contraction, and thus no high degree of accuracy was, as a rule, attainable, even if it had been possible to read off the indications sufficiently accurately;
2. The difficulty of determining the temperature of the broken-off thread of mercury at the moment its indication is read off; and
3. That the stem of the NEGRETTI & ZAMBRA Reversing Thermometers was very thick, and the graduation of the scale rough and inaccurate. It was therefore difficult to read them off accurately and withouth errors of parallax.

We took special precautions in order to avoid these errors. Specially constructed Reversing Thermometers. For the Fram Expedition 1893-96, Nansen had some NEGRETTI & ZAMBRA Reversing Thermometers specially made for more accurate observations. The distance between the division-marks of each degree was here increased, and the scale divided into 0.1° C.; but the thread of mercury had evidently thus become too thin and light, and would not, as a rule, break off when the instruments were reversed. These thermometers were therefore useless. In order to avoid these difficulties, Nansen designed new instruments with much larger bulbs, so that the degrees (Centigrade) of the scale could be made 1 centimetre long (and be divided into tenths) without reducing the diameter of the thread of mercury. These thermometers were made by NEGRETTI & ZAMBRA , but not as well as might have been desired. In order to reduce the friction between the mercury and the sides of the glass tubes, the bore of the stem was designed to be circular in section instead of elliptical (as on the ordinary reversing thermometers delivered by that firm); but this was unfortunately not done. The thermometers were also ordered to be made of Jena Normal Glass No. 16 III; but by some strange mistake only one thermometer (NZ 18) was made of this glass. This was by far the best instrument, and gave, on the whole, very satisfactory readings; but it was unfortunately lost by an accident on August 9th, 1900. Another thermometer (NZ 47) of the same shape (two other similar instruments were broken before we started) was evidently made of the same glass as NEGRETTI & ZAMBRA'S ordinary reversing thermometers. It often refused to register; and during the few weeks of the cruise of July and August, 1900, it became much worse in this respect, becoming at last perfectly useless. In order to reduce the time needed for the broken-off thread of mercury to assume the temperature of the surrounding water, when placed in a water-bath (see later), a quantity of movable mercury, or metallic sand, was enclosed within the glass sheath of the thermometers, and this would run down and surround the end of the stem (containing the broken-off mercury) as soon as the thermometers were reversed.
By comparison with the temperatures taken at the same time with the Insulated Water-Bottle, Nansen came to the conclusion that the two above-mentioned Reversing Thermometers ( NZ 18 and NZ 47 ) had changed the corrections of their zero-points from one day to another, and in his preliminary report [1901, p. 137] he has given a table of these probable changes. Upon a closer investigation of the matter, however, he has since found that this was a mistake, and that the Reversing Thermometers cannot have changed their zero-points much, the apparent variations being probably due to errors of the readings obtained by the Insulated Water-Bottle. These we shall refer to presently. Where simultaneous observations were taken with the two above-mentioned thermometers ( NZ 18 and NZ 47 ) , their corrected readings agree remarkably well.
Small Negretti & Zambra Reversing Thermometers of the old pattern. The best instruments of this type, selected from a stock of 26 thermometers, which had been used for several years, proved to register very well. One of these thermometers was used in 1900; and when read off with a specially constructed Reading-Microscope (see later), it actually gave an accuracy which might have approached the limit of ± 0.01° C., if it had not been for the rough graduation of the scale.
Water-Bath. In order to determine the temperature of the broken-off thread of mercury when the reading was taken, an arrangement was made in 1900, whereby the thermometers could be easily removed from the reversing apparatus when they came on deck. They were then placed in a water-bath, for 10 or 15 minutes before being read off with a reading- lense or a microscope. The temperature of the water-bath was taken at the same time, and entered in the journal along with the reading. The corrections made necessary by the change in the temperature of the broken-off mercury (from that which it had when reversed), were determined by experiments with each instrument.
The Reading off. The NEGRETTI & ZAMBRA Reversing Thermometers, as already mentioned, have to be read off very carefully in order to avoid serious errors. Special Reading Lenses were therefore constructed, by which it was easy to ensure the exact correspondence in level of the eye with the top of the mercury, when the reading was taken. A Reading Microscope was also designed by Nansen in 1900. It was made by Mr. LEITZ (of Wetzlar , Germany), and proved a great success. The microscope had a micrometer eye-piece, and a foot by which it could be attached to the thermometer in such a manner that its axis was always perpendicular to the thermometer-stem, and could easily be set level with the top of the mercury. By the aid of the micrometer scale of the microscope, the indications of even the small NEGRETTI & ZAMBRA Reversing Thermometers (divided into whole degrees centigrade) could easily have been read off with an accuracy of   ±0.01° C., if it had not been for their rough scale.
The Richter Reversing Thermometers. In order to get more perfect thermometers for deep-sea work, Nansen proposed to Mr. C. RICHTER, of Berlin, to try to make some improved reversing thermometers, and pointed out to him the various difficulties he had previously had with instruments of this kind. Richter agreed to make experiments, and the final result was the Richter Reversing Thermometer, which in several respects is a great improvement. This thermometer gives the temperatures with an accuracy of about ± 0.01° C. [cf. H ELLAND-HANSEN and KOEFOED, 1909].
During the first cruise the Reversing Apparatus for the thermometer was released by a propeller which could be set to reverse the thermometer, when hauled up, at any desired point within the first 15 metres of its passage upwards.
The thermometers were generally given 10 or 15 minutes to assume the temperature of the water in situ.
During the subsequent cruises releasing propellers were not used, being replaced by arrangements for messengers. The Richter Thermometers were used in 1903 and 1904, especially in connection with the EKMAN Reversing Water-Bottle, which has an arrangement for a messenger (see later) .

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Continue to  Chapter III.2.b Determination of Deep-Sea Temperature by Insulated Water-Bottles.