Johnson&rsquos Metallic Deep-sea Thermometer

The objection to the employment of mercurial thermometers for ascertaining the temperature of the ocean at depths, arising from the compression of the bulbs, which was of such serious consequence previous to the modification made in the construction of the instrument by Messrs. Negretti and Zambra, led to the construction of a metallic thermometer altogether free from liability of disturbance from compression by the surrounding water; which, however, is certainly not so sensitive to chang s of temperature as mercury. This instrument is the invention of Henry Johnson, Esq., F.R.A.S., and is thus described by him:—

“During the year 1844 some experiments were made by James Glaisher, Esq., F.R.S., on the temperature of the water of the Thames near Greenwich at the different seasons of the year; when that gentleman found that the indications of temperature were greatly affected by the pressure on the bulbs of the thermometers. At a depth of 25 feet this pressure would be nearly equal to the presence of three-fourths of an atmosphere. These observations demonstrate the importance of using in deep-sea soundings an instrument free from liability of disturbance from compression by the surrounding water, and have ultimately led to the construction of the thermometer now to be described.

“The instrument is composed of solid metals of considerable specific gravity, viz. of brass and steel, the specific gravity of these metals being 8·39 and 7·81 respectively. They are therefore not liable to compression by the water, which under a pressure of 1,120 atmospheres, or at a depth of 5,000 fathoms in round numbers, acquires a density or specific gravity of 1·06. In the construction of this instrument, advantage has been taken of the well-known difference in the ratios of expansion and contraction by heat and cold of brass and steel, to form compound bars of thin bars of these metals riveted together; and which will be found to assume a slight curve in one direction when heat has expanded the brass more than the steel, and a slight one in the contrary direction when cold has contracted the brass more than the steel.

“The indications of the instrument record the motions under changes of temperature of such compound bars; in which the proportion of brass, the more dilatable metal, is two-thirds, and of steel one-third.

“Upon one end of a narrow plate of metal about a foot long, a, are fixed three scales of temperature, h, which ascend from 25° to 100° F., and which are shown more clearly in the drawing detached from the instrument. Upon one of these scales the present temperature is shown by the pointer, e, which turns upon a pivot in its centre. The register index, g, to the maximum temperature, and the index, f, to the minimum temperature, are moved along the other scales by the pin upon the moving pointer, at e, where they are retained by stiff friction. At equal distances from the centre of the pointer are two connecting pieces, d d, by which it is attached to the free ends of two compound bars, b b, and its movements correspond with the movements of the compound bars under variations of temperature. The other ends of the bars are fastened by the plate, c, to the plate, a, on which the scales of temperature are fixed. The connection of the bars with both sides of the centre of the pointer prevents disturbance of indication by lateral concussion. The case of the instrument has been improved at the suggestion of Admiral FitzRoy, and now presents to the water a smooth cylindrical surface, with rounded ends, and without projection of fastenings.

“In surveying expeditions, this instrument would be found useful in giving notice of variation of depth of water, and of the necessity for taking soundings. A diminution of the temperature of water has been observed by scientific voyagers to accompany diminution of depth, as on nearing land, or approaching hidden rocks or shoals. Attention would also thus be attracted to the vicinity of icebergs.”

This thermometer might easily be modified to serve for several other important purposes, such as the determination of the temperature of intermittent hot springs, and mud volcanoes.

The principle of this thermometer is not altogether new; but the duplicate arrangement of the bars, which effectually prevents the movement of the indices by any shaking, and the application are certainly novel. Professor Trail, in the Library of Useful Knowledge, writes:—“In 1803, Mr. James Crighton, of Glasgow, published a new ‘metallic thermometer,’ in which the unequal expansion of zinc and iron is the moving power. A bar is formed by uniting a plate of zinc (fig. 71), c d, 8 inches long, 1 inch broad, and ¼ inch thick, to a plate of iron, a b, of the same length. The lower extremity of the compound bar is firmly attached to a mahogany board at e e; a pin, f, fixed to its upper end, plays in the forked opening in the short arm of the index, g. When the temperature is raised, the superior expansion of the zinc, c d, will bend the whole bar, as in the figure; and the index, g, will move along the graduated arc, from right to left, in proportion to the temperature. In order to convert it into a register thermometer, Crighton applied two slender hands, h h, on the axis of the index; these lie below the index, and are pushed in opposite directions by the stud, i,—a contrivance seemingly borrowed from the instrument of Fitzgerald,” a complicated metallic thermometer, described by the Professor previously.