Hugh Robert Mill

“ Graduation of thermometers, by marking off volumes of the stem equal to a given fraction of the capacity of the bulb, although reintroduced by Réaumur in 1730, has now been entirely discontinued.
The idea of a self-registering thermometer early presented itself. Many forms were devised by natural philosophers and instrument-makers. That of Sixe, in 1782, a precursor of which, dating from the 17th century, is preserved amongst the instruments of the Florentine Academy, was the most successful. ”

“ Any change of temperature displaces the centre of gravity of the system, and the instrument rotates through an arc, moving a pencil as it does so. A perfectly continuous record is produced by the photographic thermograph. Wet and dry bulb thermometers are so arranged that a beam of light passes through an air-speck, which separates part of the mercury thread, or through the vacant part of the tube, and falls on a rotating cylinder covered with photographic paper on which it traces the curve of temperature fluctuation. ”

“ The instrument is hung horizontally, and, as temperature falls, the index is drawn back by the surface tension of the fluid. When temperature rises, the liquid flows past the index easily, leaving it at the lowest point attained. Baudin invented a modification called the thermomètre à marteau in 1862; it acts vertically, the index being fixed by a spring, as in Sixe’s thermometer, and set by a long glass needle included in the stem, which, when the instrument is inverted, falls on the index and drives it to the surface of the alcohol. ”

“ The cheap German paper-scale thermometers are largely used, fitted in wooden cases, as dairy thermometers, and a larger size for brewing purposes. Alarm thermometers are often employed, in which electric contact is made and a bell rung when the temperature exceeds or falls short of a certain limit. Thermostats of various forms are made use of, in which a thermometer, by the position of the mercury in the stem, regulates the gas-supply of a burner and thus the heat of an enclosure. ”

“ No thoroughly satisfactory self-registering maximum or minimum thermometer has yet been produced. In all existing forms the indications are liable to be disturbed by shaking. Where alcohol is the fluid used, it is apt to volatilize and accumulate at the top of the tube, so registering a much lower temperature than actually occurs. It is extremely difficult also to free alcohol thermometers from air, which gradually escapes from solution in the fluid and renders the instrument untrustworthy or even useless. ”

“ Olive-shaped bulbs have been used, but a cylindrical form is most common. There should be an arrangement like that suggested for hypsometers to enable the freezing point to be verified. Casella's thermometer on Phillips’s system has a small expansion on the stem, followed by a contraction, to prevent the index following the rest of the mercury into the bulb when the instrument is not in use. The “half-minute thermometer” is quick in action; it has a bulb of very small diameter and an extremely fine bore, the mercury thread being rendered visible by Hicks’s arrangement of a lens-fronted stem. ”

“ The utmost care requires to be exercised to keep the bulb and stem dry and clean and to fill them with pure mercury recently distilled. The mercury is boiled in the thermometer for some time to drive out all traces of air and moisture, and the point of the stem is sealed off. If the thermometer is not intended to measure temperatures up to the boiling point of mercury, an expansion should be made at the top of the tube to prevent bursting from accidental overheating. ”

“ Alcohol thermometers are graduated by fixing the freezing point in melting ice and by comparison with a mercury or air thermometer at several higher and lower temperatures. Recently low-temperature thermometers have been verified at Kew in melting mercury at the temperature of -40. The law of expansion of alcohol in glass at low temperatures is not known with such precision as to make the minimum indications of Arctic expeditions entirely trustworthy. ”

“ The tube is U-shaped, the bend and part of each limb filled with mercury, the rest of the tube, the bulb, and part of the expansion on the other side with alcohol. A steel index, held in its place by the pressure of a hair, is immersed in the spirit in each limb above the mercury, which pushes one or other before it as the temperature is rising or falling, and leaves them at points denoting the highest and lowest temperatures passed through. ”

“ Richard's thermograph is also actuated by means of a sealed metallic capsule containing fluid. It draws a continuous curve in ink on a revolving drum on which one sheet lasts for seven days. This instrument is largely employed in observatories to check eye-observations, and is peculiarly adapted for use in positions to which access can only be had occasionally. It is made in many forms, one of which is specially adapted for marine work, the sealed capsule being rolled into the form of a cylinder and exposed to the water on both surfaces. ”

“ In 1721 he made a mercury thermometer according to Halley’s suggestion of 1693, and by means of it he proved the dependence of the boiling point on pressure. It was not until after Fahrenheit’s death that the freezing and boiling points of water were universally accepted as fixed points on the thermometric scale. The thermometer has remained unchanged in its main features since the middle of the 18th century. Mercury has been found the most convenient fluid for ordinary use, in spite of the advantages (Heat, vol. xi.. p. 561 ) presented by lighter and more volatile liquids. ”

“ In different countries different patterns of thermometer shelter are employed and exposure takes place at a different height above the ground. Results so obtained cannot be critically compared, and the relative mean temperatures of the atmosphere in different countries are only known to within one or two degrees. ”

“ A similar roof, but of much larger size, is employed in Australia, in combination with a metal thermometer-box. A metallic box, constructed of double louvres with an air-space between, finds favour in Spain. In Russia and Switzerland Wild’s shelter is extensively employed. The thermometers are enclosed in a case composed of two or three concentric zinc cylinders perforated to admit air, and placed 11 feet above the ground. They are protected by a large shelter of wood, the south wall and roof of which are double and made of solid boards, between which air circulates ”

“ Some form of sling thermometer should always be used for observations at sea; the Board of Trade screen generally employed is thoroughly objectionable, and can only give moderately good results by the exercise of great precautions on the part of the observer. ”

“ It is a flat spiral of brass and steel, which unrolls and coils up according to changes of temperature, moving an index on a divided horizontal circle and marking the maximum and minimum by light pointers. In order to secure regular results, the instrument must be annealed by heating for some time in boiling linseed oil.
Fig. 9.—Immisch's Avitreous Thermometer.
Several instruments known popularly as metallic thermometers depend on a different principle, that of the change of form in a thin metallic enclosure containing liquid. Immisch’s avitreous thermometer (fig. 9) is an example. ”

“ The results derived from its use are comparable, because the conditions in which it is employed are the same, but the general introduction of a double roof would greatly add to its efficiency. Exposure outside windows or in wall boxes is the rule in Austria. ”