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

by Enrico Angelo Lodovico Negretti; Joseph Zambra

As it is easily constructed and is comparatively cheap, it is still employed for ordinary purposes. Its disadvantages are, firstly, its liability of soon getting out of order by the index becoming embedded in the mercury, or fixed by oxidation, thus rendering it altogether useless; secondly, the ease with which the index can be displaced by the wind moving the instrument, or other accidental disturbance, so as to cause it to give erroneous indications occasionally; and thirdly, its consequent total unfitness for use at sea.

In the part of the tube beyond the mercury, a small quantity of air is enclosed for the purpose of preventing the metal flowing freely in the tube. This necessitates the construction of a larger bulb, which renders the thermometer less sensitive. Moreover, as it frequently happens that some mercury passes the index, particles of air insinuate themselves in the metal, and cause separations in the column, which very often can be removed only by a maker. To facilitate this re-adjustment, a small chamber is left at the end of the tube, and the mercury being expanded into it by heat until the index and air bubbles are forced into it, if possible, upon the cooling down again, by a little management, the mercury will contract, leaving the air and index behind. Yet sometimes the index cannot be moved in the least from its place of fixture, so that the instrument must be virtually reconstructed.

71. Phillip’s Maximum Thermometer.—A maximum thermometer, better perhaps in its action than Rutherford’s, has been suggested by Professor John Phillips, of Oxford. A small portion of air is introduced into an ordinary thermometer, so as to cut off about half an inch of the mercurial thread near its end in the tube. This forms a maximum thermometer, when the stem is arranged horizontally. The isolated portion is pushed forward by expansion, and is left in this position when the mercury contracts. The end remote from the bulb shows on the scale the maximum temperature.

When made with a capillary tube so fine that the attraction arising from capillarity overcomes the force of gravity, and prevents the mercury falling to the end of the tube when the instrument is inverted, it forms a very serviceable thermometer, quite portable and suitable for use on board ship. In such a tube a smart shake from a swing of the hand is required to bring the detached portion back to the column, so as to set the instrument for future observation; no ordinary motion will move it. When the thermometer has not this peculiarity, the mercury will flow to the end, if held bulb downward; and in this state it is not at all a satisfactory instrument, as the air is likely to be displaced, and a great deal of tact is requisite to again get it to divide the column suitably. It has been found in practice that the air bubble at different temperatures assumes different lengths, and if very small it disappears in a few years by oxidation and by diffusion with the mercury, so that the instrument becomes defective and uncertain in action,—results which led to the construction of the self-registering mercurial maximum thermometer, invented and patented by Messrs. Negretti and Zambra. It has been before the public about twelve years; we may therefore, now, safely speak of its merits.

72. Negretti and Zambra’s Patent Maximum Thermometer consists of a glass tube containing mercury fitted on an engraved scale, as shown in fig. 54. The part of the thermometer tube above the mercury is entirely free from air; and at the point A in the bend above the bulb, is inserted and fixed with the blow-pipe a small piece of solid glass, or enamel, which acts as a valve, allowing mercury to pass on one side of it when heat is applied, but not allowing it to return when the thermometer cools. When mercury has been once made to pass the contraction, which nothing but the expansive force of heat can effect, and has risen in the tube, the upper end of the column registers the maximum temperature. To return the mercury to the bulb, we must apply a force equal to that which raised it in the tube; the force employed is gravity, assisted when necessary by a little agitation of the instrument.

Fig. 54.

The degrees are generally divided on the stems of these thermometers, but their frames of course bear a scale as well. The makers have various styles of framing in wood, metal, porcelain, and even glass. Each material is eligible according to requirements. Porcelain scales, having the marks etched upon them by acid and permanently blackened and baked in,—by a process for which the inventors have a separate patent,—will be found very serviceable, as they do not corrode or tarnish by exposure to any kind of weather; while any amount of dust and dirt can readily be cleaned off.

The chief recommendation of this thermometer is its simplicity of construction, enabling it to be used with confidence and safety. Of no other maximum thermometer can it be said that it is impossible to derange or put it out of order; hence, as regards durability, it surpasses all others. Nothing short of actual breakage can cause it to fail. Hence it is the most easily portable of all self-registering thermometers, an advantage which renders it suitable for travellers, and for transmission abroad. In the year 1852, the British Meteorological Society reported this thermometer to be “the best which has yet been constructed for maximum temperature, and particularly for sun observations.” Since then eleven years have elapsed, and it is still without a rival.