Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/320

304 IRON high temperatures, consisting of a calorimeter into which a heated ball of platinum is dropped ; the rise in temperature of the water being noted, the amount of heat lost by the platinum, and conse quently its initial temperature, is known. The calorimeter in FIG. 19. Whitwell Stove Vertical Section. Siemens s instrument (fig. 21) is a copper cylinder jacketed outside with a double, jacket, the inner portion of which is an air space and the outer a concentric layer of hair so as to diminish errors due to radiation and atmo spheric action ; this is provided with a thermo meter, the bulb of which is protected by a copper gauze covering, fixed in a groove in the wall of the innermost vessel ; a sliding scale is attached, so constructed as to indi cate at sight the tempera ture of the ball dropped in when the zero of tho scale is adjusted to the temperature of the calori meter before starting as indicated by the height of the thermometric mer curial column, and the level of the mercury sub sequently read oil" on the scale in that position after the ball has been dropped in, the size of the ball and the quantity of water in the instrument being duly proportioned to one another. The " ball" is a cylinder of copper with a perforation in the ax;s, so that it can be lifted up on a pointed rod and introduced through a small opening into the blast main, &c., to be examined ; after a few minutes, when the ball has attained to the temperature of the blast, it is quickly withdrawn and dropped into the copper calorimeter previously filled up to a mark with water, which is well stirred up before setting the movable scale and subsequently after the ball is dropped in, so as to equalize the temperature. Usually the calorimeter holds a pint of water, the copper being of such a weight that its thermal capacity is equal to 7 V f ft pint * water. With an instrument that has been "calibrated" (i.e., the error of each scale indication determined by previous experiments at accurately measured temperatures), tho writer s experience is that very concordant and accurate results may be obtained by an expert operator working in precisely the same way as that employed in the calibration of the instrument, but that very considerable errors are apt to be introduced if the instruments as sold are employed without such corrections being made, and if the times which elapse during the withdrawal of the heated ball from the blast main and its dropping into the water are at all unequal, so that different amounts of heat are lost by atmospheric cooling before the ball reaches the water. Moreover, with -use the balls become lighter, and the temperature indicated is then too low. The other Siemens pyrometer depends on the alteration of the electrical resistance of a platinum wire when heated. The current from a suitable battery is divided into two branches, one of which Fio. 20. "Whitwell Stove Horizontal Section. passes through the experimental wire and tho other through a counterbalancing resistance consisting of another wire which is not heated, the two branches again meeting in one conducting wire which completes the circuit. If the two counterbalancing wires are at the same temperature, equal amounts of current will flow through each "branch, the resistance of the conductors, &c. , in each branch other than the two wires being equal; but if one wire be hotter, less current will How through that branch as compared with the other in proportion as the temperature is more elevated. By determining electric ally the difference between the amounts of current in the two branches, a means is afforded of calculating the temperature to which the hotter wire has been raised. In the newest form of instrument (fig. 22) a pair of ingeniously constructed voltameters is employed as the current-difference mea surer, one being in each branch of the com pound circuit, so that by reading off the volumes of gas evolved in the two, and referring to specially constructed tables, the temperature of the heated wire is at once known. In order to apply this to the mea surement of the temperature of blast mains, furnaces, &c. , the wire to be heated is wound spirally upon a porcelain cylinder, which is then enclosed in a protecting iron tube ; the ends of the platinum wire are connected with thick copper leading wires insulated by kaolin, &c. , at the hot part of the tube, and by ordinary gutta percha, &c. , at the other end. An equal amount of the same copper wire is included in the second branch, usually by carrying three wires through the hollow iron tube, one to convey the current before branching (the division taking place near the heated end), the other to convey the current through the branch containing the heated wire, and the third for the current in the other branch to pass through ; in this way errors through the unequal length and heating of the copper conductors in the two branches are avoided. Very accurate measurements are obtainable with instruments of this class when newly arranged; but it is not always certain that the resistance of a given platinum wire will remain constant after long-continued heating or interrupted exposure to high tempera tures, &c. ; in consequence it is requisite that the actual resistance after cooling of the heated wire should be verified from time to time, and the numerical values in the instrumental tables suitably adjusted when any alteration has taken place, otherwise serious errors may be introduced. For temperatures above 800 or 900 C. a peculiar pyrometer has been proposed by Lamy (Comptes Rcndus, Ixix. 347), based upon the connexion between the amount of dissociation of calcium carbonate and the pressure and temperature to which it is subjected; a glazed porcelain tube closed at one end with some fragments of marble and calcspar is filled with carbon dioxide gas and connected with a mercurial manometer ; on placing the end of the tube containing the calcspar on the furnace to be examined, the extra pressure duo to the evolved carbon dioxide is read off on the manometer, and the temperature thence deduced by a table ; on cooling, the evolved gas is reabsorbed by the partially causticized lime. Tuyeres. The heated blast passes into the furnace through nozzles or tuyeres supplied from an annular or horse shoe shaped tube carried round the lower part of the furnace at an elevation of a few feet above them by means of slant ing tubes at right angles to the ring known as " swan necks" or "goosenecks." Usually the annular tube is carried by the columns supporting the superstructure by means of suitable braces or gibbets, and communicates with the blast main at a point as near to the superheater as possible, slide valves being provided for each goose neck so that any one of the tuyeres can, when necessary, le shut off without stopping the supply of blast to the others ; when more than one furnace is supplied from the same hot main, a similar valve is provided in the branch from the main leading to each furnace. In order to adjust the nozzle of the tuyere accurately to the tuyere hole in the hearth wall, a sliding telescope joint is often inserted between the nozzle and the end of the goose neck, the lowest portion of the nearly vertical part of which is provided with a tubulus closed with a hollow stopper, the hollow of which is covered with a plate of mica; by looking through