Mercurial Minima Thermometers Desirable

Alcohol does not expand equally for equal increments of heat, consequently errors are likely to exist in the scale indications unless the graduations are very accurately—not necessarily equally—made. On this account, as well as from the volatility of alcohol, and the intervention of gaseous partitions in the tube, a good and thoroughly reliable minimum thermometer was for a long time a desideratum. It was desirable to obtain a thermometer which should register the lowest tem erature by mercury, the fluid in general use for meteorological thermometers. Several instruments have recently been invented to meet this requirement, which are suitable and satisfactory for land purposes, but one well adapted for use on board ship is still very much wanted.

For very low temperatures, alcohol thermometers will always be required; as mercury freezes at -40° F, and contracts very irregularly much before this point, while alcohol has never yet been frozen.

Negretti and Zambra’s Patent Mercurial Minimum Thermometer, represented by fig. 57, has a cylindrical bulb of large size, which, at first sight, might induce the idea that the instrument would not be sufficiently sensitive; but as length is given to the cylinder instead of increasing its diameter, it will be found as sensitive as a globular bulb of the same diameter, and much more so than an ordinary alcohol thermometer.

The reason for having the bulb large is to allow the internal diameter of the thermometer tube to be larger than that generally used for thermometrical purposes, so that a steel index, pointed at both ends, may move freely within when required.

The tube is blown, filled and regulated in the usual way, 60° of temperature being about half-way up the tube. A small cylindrical bulb is then formed at the upper end of the tube, and then is introduced a steel needle pointed at both ends, that in contact with the mercury being abrupt, the other more prolonged. The open extremity of the tube is now drawn out into a fine capillary tube, and the bulb of the instrument warmed so as to cause the mercury to fill the tube completely. When the mercury reaches the capillary tube, the flame of a blow-pipe is applied; the glass is dexterously melted, the superfluous part taken away, and the tube left hermetically closed. During this operation, the steel index has been embedded in the heated mercury. As the instrument cools, if held upright, the mercury will recede and expose the needle, which will then follow the descending column simply by its own gravity. In this condition the thermometer resembles Rutherford’s maximum, being a tube of mercury with a steel index floating on its surface; but it possesses these important advantages: it is quite free from air, so that the mercury can move with perfect freedom; and the index is pointed at both ends, to allow the mercury to pass, instead of being ground flat to prevent it.

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.