A Treatise on Meteorological Instruments / Explanatory of Their Scientific Principles, Method of Construction, and Practical Utility

by Enrico Angelo Lodovico Negretti; Joseph Zambra

To use the Thermometer, it is suspended perpendicularly (figure 57) with the steel index resting on the surface of the mercurial column. As the mercury in the cylinder contracts, that in the tube descends, and the index, of its own gravity, follows it; on the contrary, as the mercury expands and rises in the tube, it passes the index on one side, and in rising, exerts a lateral pressure on the needle, and jams it to one side of the tube, where it remains firmly fixed, leaving the upper point of the needle indicating the minimum temperature. In this thermometer, the reading is always from the upper point of the needle, and not from the mercury itself.

To extricate the Needle from the mercury, a magnet is used, when, if the needle is embedded only a few degrees, it can readily be withdrawn without altering the position of the instrument. Should the magnet not be sufficient for the purpose, we simply turn the thermometer on its support from the upright position, slightly elevating the bulb (fig. 58 (2)). The mercury and index will then flow into the small reservoir. Should the index not freely leave the tube with the mercury, assist it with a magnet, and when the mercury and index are in the upper bulb (figure 2), apply a magnet outside, which will attract and hold fast the index; and whilst thus holding it, again bring the thermometer to the upright position, when the mercury will immediately fall back into the tube, leaving the index attached to the magnet (figure 4), with which it is guided down to the surface of the mercury, ready for another observation.

Care must be taken not to withdraw the magnet until the index is in contact with mercury; for, if released before touching, it might plunge too deeply, and give a false indication. The rule for re-setting it will be to bring the needle-point in contact with the mercury, and then withdraw the magnet, having previously ascertained that no particles of mercury are attached to the index.

It may sometimes, though rarely, happen, that from the time a minimum temperature is registered by the index, and by the time an observation is made, the mercury may have risen so high in the tube as to completely pass the index, as shown (figure 3). Should it so happen, the space which the index occupies will readily be observed, as it will be pressed to one side of the tube, causing a different appearance in that part, although the point of the needle may not be seen. If such be the case, apply a magnet to the spot where you see the index is fixed: this will hold the needle firmly. Then, by slightly tilting the thermometer bulb uppermost, the mercury will flow into the top bulb, leaving the index attached to the magnet, and quite uncovered. Having taken the reading, draw the needle into the top bulb, and hold it there whilst you adjust the thermometer by again bringing it to the upright position.

By contracting the bore of this thermometer, at the bend of the tube, sufficiently to keep the mercury from flowing out of its bulb with too much freedom by motion, the instrument becomes perfectly safe for transmission abroad.

78. Negretti & Zambra’s Second Patent Mercurial Minimum Thermometer.—In this thermometer a principle is used that has been long known to scientific men, viz. the affinity of mercury for platinum. If mercury be placed in contact with platinum under ordinary circumstances, no effect will take place; but if the mercury is once made to attack the platinum, the amalgamation is permanent and the contact perfect, so much so, that the principle was made use of in constructing standard barometers. A ring of platinum was fused round the end of the tube, dipping into the mercury; and the contact between the platinum and mercury became so perfect that air could not creep down the tube and up the bore, as in ordinary barometer tubes. This principle of adhesion or affinity of mercury for platinum has been brought into play for the purpose of arresting the mercury after it has reached the minimum temperature in a thermometer. This thermometer is made as follows:—behind the bulb is placed a supplementary chamber; in the space or neck between the bulb of the thermometer and the chamber, is placed a small piece of platinum; this may be of any shape or size, but the smaller the better. This is not to fit in the neck; it must, on the contrary, be rather loose; it may be fastened in position or not. The instrument is represented by fig. 59.

Fig. 59.

Directions for using.—Having suspended the thermometer in a horizontal position, the mercury is made to stand in exact contact with the platinum plug by slightly elevating the bulb end of the instrument. The thermometer is now ready for observation. On a decrease of temperature, the mercury will endeavour to contract first from the easier passage, viz. behind the bulb; but in consequence of the adhesion of the mercury to the platinum, it cannot recede from here, it is therefore forced to contract from the indicating tube, and will continue to do so as long as the temperature decreases; and as no indices are employed in this thermometer, the extreme end of the mercurial column will show “how cold it has been.” On an increase of temperature the mercury will glide over the platinum plug and expand by the easier passage into the supplementary chamber, and there remain until a decrease of temperature again takes place, when the mercury that had gone into the supplementary chamber will be the first to recede, until it reaches the platinum plug, its further progress being arrested; it will then fall in the indicating tube, and there remain until re-set.

79. Casella’s Mercurial Minimum Thermometer.—The general form and arrangement of this instrument is shown in fig. 60. A tube with large bore, a, has at the end a flat glass diaphragm formed by the abrupt junction of a small chamber, b c, the inlet to which at b is larger than the bore of the indicating tube. The result of this is that on setting the thermometer, as described below, the contracting force of the mercury in cooling withdraws the fluid in the indicating stem only; whilst on its expanding with heat, the long column does not move, the increased bulk of mercury finding an easier passage into the small pear-shaped chamber attached.