CROMETER.
Let us now suppose that the temperature of the two cast iron prismatic bars is kept unaltered, which can be tasily ascertained by keeping a thermometer in the water of the troughs in which each is placed. Let the first mi croscope with a joint in its field be set above one end of the metallic bar to be examined, so that this point or mark is directly coincident with the extremity of the me tallic bar, or with a point or mark near its extremity. In like manner, let the second microscope be fixed above the other extremity of the metallic bar, so that its move able wire is exactly coincident with the other extremity of the bar, or with a point or mark near it. When these adjustments are made, let us suppose that the tempera ture of the water in the copper trough, and consequently the temperature of the metallic bar itself, is exactly 50°. Place the twelve spirit lamps beneath the copper trough, ,so as to raise the temperature of the water in it, and con sequently that of the bar gradually to 150° or any other temperature; and look through the second micrometer microscope. From the instant that the bar begins to ex perience the expansive action of the heat, the mark at its extremity will be seen to lose its coincidence with the wire of the micrometer, and extend beyond it. At any temperature, therefore, namely, 70°, 80°, we can measure the expansion that has taken place by observing bow many turns and parts of a turn of the mierome.er screw are necessary to bring the micrometer wire into coinci dence with the slowly moving mark on the expanding bar; and as the value in parts of an inch of each turn of the micrometer screw is accurately known, we obtain a direct measure of the elongation of the bar, free from all the errors of wheels, levers, and pinions. By this instru ment, General Roy obtained the expansions of the bodies to which his name is affixed under EXPANSION. See Phil. Trans. 1785, vol. lxxv. p. 462.
8. On Trough!on's Pyrometer.
In our article EXPANSION, we have mentioned the mi crometer of that able artist, Mr. Trougliton, and promised a description of it under the present article. Mr. Trough ton constructed this instrument in 1794, on a small scale, for trying pendulums, and we believe that he does not wish any drawing or minute descri,tion of it ; ublished till he has corn, leted the large instrument which he has long planned. for measuring the expansion of bars ten feet in length. We may mention, however, that this pyrome ter measures by the indication of a fine level, which is made to deviate from a horizontal position by the direct influence of the elongation of the bar.
9. Description of Dr. Brewster's Chromatic Pyrometer.
In the different instruments that have been described, the actual elongation of the metallic bar is either magni fied by levers, or by wheels and pinions, or it is measured directly by a micrometer screw attached to a compound microscope. In the present instrument, however, the elongation of the bar is measured by the number or the intensity of the polarised tints, which it produces by the inflexion of a !late of glass. If AB, plate CCCCLXXI. No. 111. Fig. 8, for exam; le, is the metallic bar at a given temperature, whose end A touches the surface of the glass-plate P Q, then, when it ex.nands by heat, the end A will bend the glass plate into a curve, the sagitta of which is equal to the elongation; and by the principles already folly detailed in our article OPTICS, the polarised tints corresponding to the degree of inflexion of 0 P, and consequently to the length of the sagitta of curvature in P Q, or the elongation of the bar, will be seen by look ing through the edge P Q with a polarising and analysing apparatus. The tints thus produced have an accurate numerical value, and the scale by which they measure the elongation, may be varied by altering the thickness and length of the plate of glass P Q.
There is one form of this instrument which seems to have a peculiar application to the use generally made of pyrometers for horological purposes. Let us suppose that the clock maker wishes to have the exact length of a bar of zinc, which has the same expansion as a bar of brass of a given length, for the purpose of destroying the ef fects of the expansion by their opposite action The ends of the two bars cannot be made to press on opposite sides of the plate of glass without breaking it, hut by using these plates, as shown in Fig. 8, where XZ is a plate of glass placed between two plates MN, PQ, which may either be of glass or metal of equal thickness. If AB is the bar of zinc, and CD the bar of brass, whose expan sions are to he equalised, each of them will produce the same inflexion in the plates MN, PQ, which, if they are made of glass, will show either equal or unequal tints; or the middle plate XZ will show no tints at all, if the expansio'ns of the metallic bars are equal. If the curva ture of one of these plates predominates, the plate XZ will he concave towards that plate, and this concavity, though too minute to be ascertained by the eye, will be rendered obvious by the positive or negative character of the polarised tints on each side of the neutral line.