PYROMETER

, or fire-measurer, a machine for measuring the expansion of solid bodies by heat.

Musschenbroek was the first inventor of this instrument; though it has since received several improvements by other philosophers. He has given a table of the expansions of the different metals, with various degrees of heat. Having prepared cylindric rods of iron, steel, copper, brass, tin, and lead, he exposed them first to a Pyrometer with one flame in the middle; then with two flames; then successively with three, four, and five flames. The effects were as in the following Table, where the degrees of expansion are marked in parts equal to the 12500th part of an inch.

Tin easily melts when heated by two flames placed close together; and lead with three flames close together, when they burn long.

It hence appears that the expansions of any metal are in a less degree than the number of flames: so two flames give less than a double expansion, three flames less than a triple expansion, and so on, always more and more below the ratio of the number of flames. And the flames placed together cause a greater expansion, than with an interval between them.

For the construction of Musschenbroek's Pyrometer, with alterations and improvements upon it by Desaguliers, see Defag. Exper. Philos. vol. 1, pa. 421; see also Musschenbroek's translation of the Experiments of the Academy del Cimento, printed at Leyden in 1731; and for a Pyrometer of a new construction, by which the expansions of metals in boiling fluids may be examined and compared with Fahrenheit's thermometer, see Musschenb. Introd. ad Philos. Nat. 4to, 1762, vol. 2, pa. 610.

But as it has been observed, that Musschenbroek's Pyrometer was liable to some objections, these have been removed in a good measure by Ellicott, who has given a description of his improved Pyrometer in the Philos. Trans. numb. 443; and the same may be seen in the Abridg. vol. 8, pa. 464. This instrument measures the expansions to the 7200th part of an inch; and by means of it, Mr. Ellicott sound, upon a medium, that the expansions of bars of different metals, as nearly of the same dimensions as possible, by the same degree of heat, were as below:

The great difference between the expansions of iron and brass, has been applied with good success to remove the irregularities in pendulums arising from heat. Philos. Trans. vol. 47, pa. 485.

Mr. Graham used to measure the minute expansions of metal bars, by advancing the point of a micrometer screw, till it sensibly stopped against the end of the bar to be measured. This screw, being small and very lightly hung, was capable of agreement within the 3000 or 4000th part of an inch. And on this general principle Mr. Smeaton contrived his Pyrometer, in which the measures are determined by the contact of a piece of metal with the point of a micrometer screw. This instrument makes the expansions sensible to the 2345th part of an inch. And when it is used, both the instrument and the bar, to be measured, are immerged in a cistern of water, heated to any degree, up to boiling, by means of lamps placed under the cistern; and the water communicates the same degree of heat to the instrument and bar, and to a mercurial thermometer immerged in it, for ascertaining that degree.

With this Pyrometer Mr. Smeaton made several experiments, which are arranged in a table; and he remarks, that their result agrees very well with the proportions of expansions of several metals given by Mr. Ellicott. The following Table shews how much a foot in length of each metal expands by an increase of heat corresponding to 180 degrees of Fahrenheit's thermometer, or to the difference between the temperatures of freezing and boiling water, expressed in the 10000th part of an inch.

For a farther account of this instrument, with its use, see Philos. Trans. vol. 48, pa. 598.

Mr. Ferguson has constructed, and described a Pyrometer (Lect. on Mechanics, Suppl. pa. 7, 4to), which makes the expansion of metals by heat visible to the 45000th part of an inch. And another plan of a Pyrometer has lately been invented by M. De Luc, in consequence of a hint suggested to him by Mr. Ramsden: for an account of which, with the principle of its construction and use, both in the comparative measure of the expansions of bodies by heat, and the measure of their absolute expansion, as well as the experiments made with it, see M. De Luc's elaborate essay on Pyrometry &c, in the Philos. Trans. vol. 68, pa. 419— 546.

Other very nice and ingenious contrivances, for the measuring of expansions by heat, have been made by Mr. Ramsden; which he has successfully applied in the case of the measuring rods and chains lately employed, by General Roy and Col. Williams, in measuring the base on Hounslow Heath, &c; which determine the expansions, to great minuteness, for each degree of the thermometer. See Philos. Trans. 1785, &c.