A pyrometer of another kind has been suggested by Dr. Brewster. The bar AB presses against a piece of metal AC, which has its face AC ground with emery. A small metallic, or glass cylinder D, truly ground into a socket, in which it freely moves, is turned round by the action of the face AC, which is kept against it by a spring. Instead of putting an index and graduated plate upon D. it is connected with the axis of a goniome ter, and its angle of rotation measured with great accu racy. The angle thus found will be an exact measure of the arch of rotation of the surface of the cylinder, and con sequently of the elongation of the bar.
ALTHOUGH the instruments which we propose to deAlthough the instruments which we propose to de- scribe under this chapter, are only thermometers adapted for measuring degrees of heat far above the indications of the common thermometer, yet as they are in reality different instruments, founded on different principles, and distinguished by a different name, we shall not scruple to introduce them here.
1. Description of Wedgewood's Pyrometer.
In our article CHEMISTRY, we have already given a general description of this instrument, and of the princi ples on which it is founded. We have, therefore, only to give a drawing of the gauge and an account of the relation of its scale to the common thermometrical scale of Fahrenheit.
In plate CCCCLXX1. No. 111. Fig. 9 which represents the pyrometrical gage, ABCD is a smooth flat plate, and and GH two rulers, or flat pieces, a quarter of an inch thick, fixed upon the plate, with the sides that are towards each other made perfectly true; and a little far ther distant at one end EG than at the other end FH, so as to include a long converging canal, which is divided on one side into a number of small equal parts. Now, if a body I is so adjusted as to fit exactly at the narrow end FF1 of the canal, so that its lower side stands ex actly at 0° ; then, if it is expanded by heat, and ap plied in that expanded state to the scale, it will no longer fit into the narrow end FII, but will fill a wider part of the scale farther up, and the divisions opiosite to the lower side of 1, when it fits, will indicate the de gree of expansion. In like manner, the degree of con traction will be obtained by the reverse operation. The following Table, given by Mr Wedgewood, shows the relation between his pyrometer and the thermometer of Fahrenheit.
2. Description of Guyton's Platina Pyrometer.
The object of this instrument, which was exhibited to the National Institute, in 1803, was, to measure the highest degree of heat in our furnaces. It consists of a rod or plate of platina, placed horizontally in a groove, formed in a cake of hardened white clay. This plate is supported, at one of its extremities, on that part of the mass which terminates the groove, while the other ex tremity presses against a bent lever, whose longest arm forms an index to a graduated arch, so that the expansion is indicated on the scale, by the index. The bar 01 platina
was an inch and three quarters long, a quarter of an inch wide, and one-thirteenth of an inch thick. The arm of the bent lever, against which the platina bar presses, is one-ninth of an inch; and the arm at right angles to it, which acts on the index, one inch and eight-tenths, or twenty times as long. The index carries a nonius, which divides each degree into ten parts; hence Guyton cal culates that we may measure an expansion of the 5730 th part of the radius In order to prevent the position of the index from being changed, in removing the instrument from the furnace, a plate of platina is fixed so as to form a spring against its extremity. See the Annales de Chimie, No 138. vol. xlvi. p. 276, and Nicholson's Philosophical Journal, vol. vi. p. 89.
3. Description of Mr Daniell's Platina Pyrometer.
The pyrometer of Mr. Wedgewood, ingenious as it is, had nevertheless fallen into disuse, both from the extreme difficulty of procuring pieces of clay of uniform compo sition, and from its having been found that time has an influence on the contraction of the clay pieces, the longer continuance or a low degree of heat producing the same contraction as a higher degree of heat continued for a shorter time.* Air Daniell was, therefore, led to construct a pyrometer free from these defects, and which promises to be of much utility in the arts, as well as in the sciences. It is repre sented in Plate CCCCLXXI. No. III. Fig. 10 and 11; Fig. 10 representing the whole instrument, and Fig. 11 a part of had the real dimensions. The tube a b c is made of black lead earthenware, and the shoulder in its centre is moulded when it is formed. The end a of the tube is close, and the end c, which is open. is accurately fitted into a ferule of brass d, which carries the scale e f g h. Within the tube a 6 is a bar of platinum 10 2 inches long, and of an inch in diameter, extending from 6 to a, where it is fixed by a nut and screw of the same metal on the outside, and a pin or shoulder on the inside. At 6 it is kept in its place by a small perforated plate of pla tinum, through which it passes. A fine platinum wire, about 100th of an inch in diameter, proceeds from the end 6 of the platinum wire, and coming out of the tube at d, a piece of silk thread attached to it, is coiled twice or thrice round the axis of the wheel 1, fixed on the back of the scale, and shown in Fig. 11. The thread is then turned back, and attached to the extremity of a slight spring m n, which is stretched on the outside of a brass ferule, and fixed by a pin at n. By this means, the action of the spring keeps the wire in a state of tension. The axis of the wheel is of an inch in diameter, and the wheel itself, which is toothed, works in the teeth of another smaller wheel k, one third the diameter of the other, and having one-third of the number of teeth. An index 1 is fixed to its axis, which passes through the centre of the scale e f g h, which is divided into 360 degrees.