Charles R. Darling

“ In cooling down a mass of steel the fall of temperature is arrested at one or more points, observations of which are frequently of service in deciding the subsequent treatment of the steel. A method commonly employed is known as the “differential method,” and is indicated in fig. 28. The sample of steel, A, is placed side by side with a piece of nickel, B, of equal dimensions, in the tube of an electric furnace. A naked junction, C, is placed in a hole drilled in A, and is connected to the galvanometer G, which is calibrated to read temperatures. ”

“ The degrees were, of course, arbitrary; but with cylinders of uniform make a given position in the groove after heating always represented the same furnace temperature, and thus furnished an indication more reliable than the judgment of a workman’s eye. Wedgwood attempted to express the divisions on his scale in terms of Fahrenheit [213] degrees, and by extrapolation of results obtained at the highest limits of the mercury thermometer, where 1 degree of contraction was caused by a rise of 130° F., arrived at figures which now appear ludicrous, but which were accepted for forty years. ”

“ It was not until 1886 that the problem was satisfactorily solved by Le Chatelier of Paris.
Although any heated junction of metals will give rise to an electromotive force, it does not follow that any pair, taken at random, will be suited to the purposes of a pyrometer. A junction of iron and copper, for example, gives rise to an E.M.F. which increases with the [21] temperature up to a certain point, beyond which the E.M.F. falls off although the temperature rises, and finally reverses in direction—a phenomenon to which the name of “thermo-electric inversion” has been applied. ”

“ A temperature scale is provided on the moving piece which actuates the wedges, and is derived by Wien’s equation from the thickness of the wedges interposed when equality is obtained. Calibration is effected by noting the thickness of the wedges corresponding to two known temperatures, from which a straight line connecting thickness with the reciprocal of the absolute temperatures may be drawn, and a table formed giving values of T in terms of the thickness of the wedges. The calibration may be extended indefinitely, the accuracy of the readings depending upon the truth of Wien’s law. ”

“ As an instrument of research in the laboratory, a good form of optical pyrometer is very useful, as, for example, in investigating the working temperatures of electric lamps, [194] and taking observations in electric furnaces. It is a great drawback that records cannot be taken by optical pyrometers, as much valuable information can be gathered from an accurate knowledge of temperature fluctuations in most operations. This disadvantage must always militate against the general use of these instruments. ”

“ If a block of steel be sighted inside a furnace, and then be removed to the exterior and again sighted, the external reading will be much less than the internal, owing to the inferior radiating power of the surface, which now derives no assistance from the furnace. All readings should therefore be taken whilst the object is still in the furnace, or (as in taking the temperature of molten metal in a ladle) a fireclay tube with a closed end inserted in the mass may be used, and readings taken through the open end. ”

“ An agreement on this point between instrument makers would overcome the difficulty at once, as the Centigrade scale is now so widely used that few purchasers would insist on Fahrenheit markings. [8]
It may be pointed out here that no single pyrometer is suited to every purpose, and the choice of an instrument must be decided by the nature of the work in hand. A pyrometer requiring skilled attention should not be entrusted to an untrained man; and it may be taken for granted that to obtain the most useful results intelligent supervision is necessary. ”

“ If a perfectly constant temperature were to be measured, a resistance pyrometer would undoubtedly give a closer indication; but constancy to 10° C. is seldom possible with gas-fired or coal furnaces or other hot spaces in which pyrometers are used. The accuracy of a pyrometer under workshop conditions therefore depends upon the rapidity with which it responds to temperature fluctuations, which condition will evidently be influenced by the thermal conductivity of the sheath. ”

“ Taking the case of a block of steel in a furnace, it is possible to arrange combinations of glasses which, when illuminated by a standard light, will give the same tint as the steel at any specified temperature. If it be desired to work the steel at 850° C., for example, glasses are provided which, when viewed by the light transmitted from a 4-volt glow-lamp, using a constant current, represent the tint of steel at 840°, 850°, and 860° respectively. ”

“ If leads of considerable length were used to connect the pyrometer with the indicator, and were subject to fairly large alterations of temperature, the consequent changes in the resistance of such leads would be noticeable on a low-resistance indicator; and similarly, if a pyrometer were inserted at different depths in a furnace at separate times, thus heating up varying lengths of the junction wires, a discrepancy would arise for the same reason. The resistance of an indicator, however, cannot be raised beyond a certain point without reducing the sensitiveness below the required limit. ”

“ General Principles.—When a pure metal is heated, its resistance to electricity increases progressively with the temperature. Certain alloys, on the other hand, show a practically constant resistance at all temperatures, examples of such alloys being constantan, manganin, and platinoid. ”

“ The zero resistance of a given instrument depends upon the accuracy of the measuring appliances used, and upon the degree of precision it is desired to attain. If, for example, it is intended to read to 1°C., with appliances capable of measuring to 1/100 of an ohm, a convenient zero resistance is 2·6 ohms; it being found that with pure platinum the resistance rises from 2·6 ohms at 0° to 3·6 ohms at 100° C., an increase of 1/100 of an ohm for 1° C. With coarser measuring arrangements, for the same degree of precision, a correspondingly higher zero resistance will be required ”

“ Fig. 3.—Three-junction Thermo-electric Circuit.
A point of practical utility in thermo-electric work is the fact that if a wire be interrupted by a length of other metal, as indicated at C in fig. 3, no current will be set up in a circuit if both joints are equally heated, as the electromotive forces generated at each junction [26] are in opposition. It is thus possible to interrupt a circuit by a plug-key or switch, without introducing an error; always provided that an even temperature prevails over the region containing the joints. ”

“ As showing how platinum itself is not uniform, it may be mentioned that almost any two pieces of platinum wire, if not from the same length, will cause a deflection on a sensitive galvanometer when made into a junction and heated. It is therefore customary for makers to obtain considerable quantities of wire of a given kind, homogeneous as far as possible, in order that a number of identical instruments may be made, and the junctions replaced, when necessary, without alteration of the scale of the indicator. ”

“ The creeping movement is probably due to the whole instrument, and the air in the interior, becoming heated by the entering rays, and by proximity to the hot source. In a number of trials made by the author, it was noticed that when the instrument was allowed to stand near the furnace for some time before using, thereby attaining the temperature existing in the vicinity, the “creep” almost entirely vanished. All things considered, the spiral form of Féry’s pyrometer must be regarded as more portable but less accurate than that in which the rays are received on a thermal junction. ”

“ Fig. 27.—Connections for an Installation of Pyrometers.
In some instances a second indicator is kept in the shop office, to which all the pyrometers are wired, and which serves as a standard. The scale of the office indicator is checked daily at one point; and by connecting a given couple first with the shop indicator, and immediately afterwards with the office standard, any errors can be detected. It is also possible to ascertain the temperature of any given furnace in the office at any time, and so to control the whole. ”

“ The length of the spiral is such that an alteration of a given number of degrees in its temperature moves the pointer by the same number of degrees on the scale—or, in other words, the temperature scale of the pyrometer is identical with that of the spiral. The metals forming the junction are continued, in the form of wires, to the interior of the galvanometer, where a cold junction is formed, which will always possess the same temperature as the spiral. ”

“ Coating with aluminium powder also greatly prolongs the life of an iron sheath. When used in lead baths, the immersed part, if of iron or steel, should be bored from the solid, and left thick at the portion opposite the surface of the lead, where most corrosion occurs. A graphite tube, or one made of a composition containing graphite, is often useful in cases where iron is readily corroded, and can be used to much higher temperatures. ”

“ When existing instruments are compared with this standard, it is found that a scale based on the assumption that the volume of a gas free to expand, or the pressure of a confined gas, increases directly as the temperature is in close agreement with the thermodynamic scale. ”

“ Conduction Pyrometers.—If one end of a rod of metal be inserted in a furnace, heat will be conducted along it to the portion external to the furnace, and a steady condition will be obtained when the heat escaping from the external part of the rod, by convection and radiation, is equal to the quantity conducted along the rod. The hotter the portion in the furnace, the higher will be the temperature of all parts of the external length. ”

“ General Principles.—It is a common experience that the heat radiated by a substance increases as its temperature rises; and it would obviously be an advantage if the temperature of a hot body could be deduced from the intensity of its radiations, as the measurement could then be made from a distance, without the necessity of placing a pyrometer in contact with the heated substance. ”

“ When an occasional reading of temperature, true to 3 per cent., suffices, the calorimetric pyrometer may be used; and in special laboratory determinations the method will frequently be found of value. Considering the low cost of thermo-electric pyrometers at the present time, it is probable that the calorimetric method will be entirely superseded in industrial practice, as the former method gives a continuous, automatic reading, and is capable of furnishing records. Many firms have already replaced their “water” pyrometers by the more accurate and useful appliances now available. ”

“ When applied to the measurement of high temperatures, the bulb must be made of a more infusible material than glass. Gold, porcelain, platinum, and quartz have been used by different investigators, but the most reliable material for temperatures exceeding 900° C. has been found to be an alloy of platinum with 20 per cent. of rhodium. The most suitable gas to use inside the bulb is nitrogen, which is chemically inert towards the materials of the bulb, and is not absorbed by the metals mechanically. ”

“ A third method would be to focus the rays on to a compound strip of two metals, which by altering in shape could be made to furnish a clue to the quantity of energy received by it. In theory, it is only necessary to allow the radiations to fall on the working part of any instrument for measuring low temperatures, when the rise in temperature produced may be taken as proportional to the energy received, and the thermal condition of the radiating body deduced from the fourth-power law. ”

“ Foster’s Fixed-Focus Radiation Pyrometer.—The necessity for focusing, common to all Féry’s radiation pyrometers, is obviated in Foster’s pyrometer, which, however, cannot be used from so great a distance. The principle involved in the fixed-focus pyrometer is that the amount of energy received by a concave mirror and focused on a thermal junction will not vary so long as the area of the surface sending rays to the mirror, through a fixed opening, increases as the square of the distance. ”

“ Amongst the standards employed are carbon-filament electric lamps, amyl-acetate lamps, and for higher temperatures the centre of an acetylene gas-flame; each of which is capable of producing a fixed degree of brightness when used under specified conditions. A black body, at known temperatures, is compared with the standard used, thus furnishing a scale of “black-body” temperatures to which the indications of a given source may be referred, as explained in the previous paragraph. ”

“ With a mirror galvanometer it is better to move the scale, or turn the galvanometer round on its axis to restore the correct zero, rather than to twist the coil back; but with a fixed scale and pointer the only remedy is to turn the coil bodily round. In a single-pivot indicator constantly used in the author’s laboratory, the creep amounted to a movement of the end of the pointer through an angle of 2 degrees in the first few months, since when, after the lapse of several years, no further alteration has occurred. ”

“ Special Uses of Radiation Pyrometers.—For regular use at temperatures above 1000° C. or 1850° F. the radiation pyrometer will be found to be more useful than instruments of the thermo-electric or resistance type as the latter undergo deterioration owing to the continuous action of the furnace gases, which becomes more marked as the temperature increases. Examples of industrial processes in which 1000° C. is considerably exceeded are the manufacture of glass, pottery, and cement, the treatment of special steels, and the casting of metals and alloys. ”

“ Unless the necessary facilities are at hand, an operator would be well advised to standardize a resistance pyrometer by taking several fixed points and drawing a calibration curve, after the manner recommended for a thermo-electric pyrometer.
If a resistance pyrometer be calibrated so as to read in platinum-scale degrees, and the value of δ be known for the wire, the correct gas-scale temperatures may be calculated from Callendar’s formula. ”