CHAPTER IX

MISCELLANEOUS APPLIANCES

=Expansion and Contraction Pyrometers.=—Most substances, on heating, exhibit an increase in size, and on cooling return to the original dimensions. If, however, a chemical alteration occur during the heating, the resultant material may be permanently altered in size, so that on cooling the substance may be of less or greater dimensions than before. Both these phenomena have been applied to the measurement of high temperatures; the permanent shrinkage undergone by clay being utilised by Wedgwood in the instrument which was the first practical pyrometer; the expansion of a solid by Daniell, and of liquid by Northrup. Both forms are still in use to a limited extent, and will now be described.

=Wedgwood’s Pyrometer.=—In 1782 Wedgwood introduced a method of determining the condition of a furnace by observing the contraction shown by cylinders prepared from a special clay. The measuring device took the form of a tapered groove (fig. 68) made in two parts, each 6 inches long, and one a continuation of the other. Each inch of the groove was divided into 20 equal parts, making 240 divisions in all, and each division was called 1 degree. The width of the groove opposite the zero mark was 0·5 inch, and opposite 240, 0·3 inch. Before firing, the cylinders entered the groove until the lower end was opposite or near the zero mark; and after being inserted in the furnace and allowed to cool on removal, the cylinders were pushed as far as possible down the groove, when the mark opposite the lower end indicated the condition of the furnace in terms of Wedgwood’s scale. 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 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. As examples, the melting point of silver was given as 4717° F.; of cast iron, 17977° F.; and of wrought iron, 21637° F.—the last figure being nearly 19000° higher than the present accepted value of 2770° F. The error arose from the assumption of uniform contraction with increase of temperature, and furnishes a striking example of the danger of indefinite extrapolation from meagre data. But although the expression of the result in Fahrenheit degrees was so erroneous, the observed contraction always corresponded to a given condition of the furnace, and the firing was continued until that known to be the best for the work in hand was attained.

The permanent shrinkage referred to is caused by dehydration of the clay, and it therefore follows that this method can only give uniform results when exactly the same kind of clay is used for the test-pieces. A given manufacturer might secure consistent indications by making a quantity of clay, to be kept specially for this purpose; but the same contraction at a given temperature would not be obtained by a second observer who also had prepared a quantity of clay, as slight differences in composition cause large variations in the observed contraction. In practice, therefore, pyrometers of this type are not interchangeable, and each user must standardize for his own special conditions. Wedgwood’s pyrometer is still used to a small extent; its replacement, however, by the more convenient and accurate instruments now available is only a question of time.

=Daniell’s Pyrometer.=—In 1822 Daniell published an account of a pyrometer based on the expansion of a platinum rod enclosed in a plumbago tube. One end of the rod pressed against the end of the tube, whilst the other end was free to move, and was connected to a multiplying device which magnified the expansion, the increased movement being indicated by a pointer, moving over a dial. The scale on the dial was divided evenly into a suitable number of parts, it being assumed that the difference between the expansions of graphite and platinum was uniform at all temperatures. The scale was calibrated as far as possible by comparison with a mercury thermometer, the remainder being extrapolated. With this pyrometer Daniell obtained a value of 2233° F. for the melting point of silver, and 3479° F. for that of cast iron—results considerably higher than those now accepted, but much nearer than those obtained by Wedgwood. Daniell’s pyrometer was widely used, and its modern representatives are fairly common. Platinum, owing to its cost, is no longer used in these instruments, which are now generally made with a graphite rod encased in an iron tube, on the end of which the graduated dial is placed, as shown in fig. 69. Another form, commonly used in baker’s ovens, is constructed with an iron rod surrounded by a porcelain or fireclay tube.

The defect of pyrometers of this type is that the coefficient of expansion of the materials alters with prolonged heating, causing the readings to become erroneous. Re-adjustment in boiling water or other substance does not compensate properly for this alteration, as both materials are not equally affected. Again, the readings will be too low unless the whole of the expanding parts are in the interior of the furnace, in which respect this pyrometer is inferior to a thermo-electric instrument, which may be inserted at any convenient depth, and may therefore be used for a greater variety of purposes. The chief recommendation is cheapness; but an expansion pyrometer should never be used for work of precision. A graphite rod in an iron enclosure gives more consistent results than other materials.

=Northrup’s Molten Tin Pyrometer.=—Tin melts at 232° C., and boils at 2270° C. It does not give off vapour sensibly up to 1700° C., and expands with great uniformity. It is therefore suitable for measuring high temperatures on the same principle as an ordinary thermometer, and Dufour, in 1900, attempted to make a high-reading thermometer by enclosing tin in a silica bulb. Northrup has constructed an instrument in which the bulb and stem are of graphite, and the height of the molten tin is determined by lowering a nickel wire through a gland until it touches the tin, thereby completing an electric circuit and causing a bell to ring or producing a deflection on a galvanometer. The upper end of the nickel wire moves over a scale, which may be marked at two suitable fixed points, and the scale divided up as in the case of an ordinary thermometer. The durability of the graphite cover will determine the utility of this pyrometer, and protection by some good refractory will be essential to prevent oxidation. Such a pyrometer will not respond quickly to changes in temperature, but may prove useful in reading temperatures at ranges beyond the scope of present thermo-electric pyrometers. Northrup anticipates that this instrument may be used up to 1800° C.

=Vapour-Pressure Pyrometers.=—In these instruments mercury is placed in a stout steel tube, to which a pressure-gauge is attached, which registers the vapour-pressure of the mercury. Readings of pressure may be translated into temperatures by calibration with a standard pyrometer. The range of these instruments is limited—600° or 700° C.—and they are seldom used at present, having been superseded by more modern types.

=Water-Jet Pyrometers.=—In these instruments water is passed through a pipe placed in the furnace or hot space at a definite rate, and from the rise in temperature produced in the water that of the furnace may be obtained. An outfit of this kind entails the provision of a steady source of water-pressure, and the indications can only remain accurate so long as the bore of the pipe remains uniform. The calibration is made by comparison with a standard pyrometer. The drawbacks to the method are its inconvenience, and the necessity for continuous skilled supervision; and in consequence of these the arrangement is seldom used.

=Pneumatic Pyrometers.=—Attempts have been made to deduce furnace temperatures by blowing air at uniform pressure through a pipe located in the hot space, and noticing the increase in the temperature of the air. In the Uehling pyrometer, air from the hot space is drawn through an opening of fixed size by means of a steam-jet, which acts as an aspirator. The opening is placed at one end of a chamber, and the steam-jet aspirator at the other end; and a diaphragm with a central hole divides the chamber into two parts. The pressures existing in the two portions of the chamber vary according to the temperature of the air drawn in, and are measured by water-gauges, the readings of which may be translated into temperatures by calibration against a thermo-electric or other pyrometer. The method is ingenious, but is elaborate and costly; and is therefore little used.

=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. A series of thermometers placed at intervals in the exterior portion would show a progressive fall in temperature along the rod; and the hotter the furnace the higher would be the reading on each thermometer. In applying this principle to the measurement of high temperatures, a bar of copper or iron is passed through the wall of the furnace, so that a length of 2 feet or more protrudes on the outside. Near the end of the external portion a hole is drilled to a sufficient depth to cover the bulb of a thermometer, which is inserted in the hole, into which a quantity of mercury is poured to make a metallic contact between the bulb and the bar. The reading of the thermometer furnishes an approximate clue to the temperature of the furnace, rising or falling with corresponding changes in the hot space. A calibration might be effected by comparison with a standard; but the method is only applied to the production of a prescribed condition, known by experience to be attained when the thermometer reading has a certain value—say 120° C. Changes in atmospheric temperature, or currents of air, seriously affect the readings, and the method at best is only approximate.

=Gas Pyrometers.=—Wiborgh, Bristol, and others have constructed pyrometers in which the pressure of an enclosed gas is recorded by a Bourdon pressure-gauge, the scale of which is calibrated so as to read temperatures. A porcelain bulb, terminating in a capillary tube which is connected to the gauge, is used to contain the air or other gas; but at temperatures above a red heat the readings become uncertain, owing either to leakages or the distortion of the bulb. The most suitable material for the bulb (alloy of platinum, 80 per cent., and rhodium, 20 per cent.) is too costly to use industrially, and would deteriorate under the influence of furnace gases. In the Bristol recording instrument the moving index of the pressure-gauge terminates in a pen, which touches a chart-paper revolving by clockwork. Good results are obtained up to 400° C., but beyond this the indications are uncertain, and the instrument is more correctly described as a recording thermometer.

=Wiborgh’s Thermophones.=—These consist of infusible clay cylinders, 2·5 cms. long and 2 cms. in diameter, which contain an explosive. When placed in a hot space, the explosion occurs after a definite time, the interval being less at high temperatures than at lower, as the rate at which heat is conducted through a solid varies directly as the difference between the external and internal temperatures. The interval elapsing between placing in the furnace and the subsequent explosion is noted on a stop-watch to the nearest 1/5 second, and from the observed time the temperature is obtained from a table, drawn up from the results of experiments under known conditions. If the cylinders be kept dry, an observer experienced in the use of thermophones may secure a reading to within 40° C.

=Joly’s Meldometer.=—This device, due to Dr Joly, is intended for laboratory determinations of melting points. It consists of a strip of platinum, heated by electricity, upon which a tiny fragment of the material is placed, which is viewed through a microscope. The temperature of the platinum is regulated by means of a rheostat in the circuit, and in making a test the temperature is gradually raised until the material is observed to become globular, or to flow over the platinum strip. The temperature at which this occurs is deduced from the linear expansion of the platinum strip, which is measured by a micrometer attached to the instrument. When carefully used, very accurate determinations may be made by the meldometer, the results, moreover, being obtained rapidly, and with the use of the minimum of material.

=Brearley’s Curve Tracer.=—This apparatus made by the Cambridge and Paul Instrument Company, is designed to take a large-scale record of an operation which only occupies a short period of time. It consists of a drum, round which the record paper is wound, and capable of rotating on its axis once in ten or thirty minutes by the aid of clockwork. Attached to the arm of the pen is a pointer, which moves along the scale of a sensitive mirror galvanometer to which a thermocouple is connected. The operator, by turning a handle, moves the drum longitudinally so as to keep the pointer opposite the centre of the spot of light, and this movement is traced on the chart, combined with the rotary movement, by the pen. In this manner the large change in deflection, due to a few degrees increase or decrease in temperature, can be recorded in ink. This instrument is of special service in recording the critical points of steel, or any operation which involves delicate readings over a limited range of temperature.

Index

Air pyrometers, 219. Anti-vibration stand for galvanometers, 47. Atmospheric temperatures, measurement of, 96, 132. Automatic compensators, 66.

_Barrett, Sir W._, discovery of recalescence, 4. _Becquerel, Ed._, optical pyrometer, 5. Black-body radiations, 136. _Brearley_, sentinel pyrometers, 208. — curve-tracer, 221. _Bristol_, air-recording pyrometer, 219. — compensator, 66. _Byström_, calorimetric pyrometer, 201.

Calorimetric or “water” pyrometers, 195. — — — special uses of, 203. Centigrade scale of temperatures, 6. Clay-contraction pyrometers, 211. Colour-extinction pyrometers, 189. Colour, in relation to temperature, 167. Comparison of gas and platinum scales, 111. Compensators for cold junctions, 66. Conduction pyrometers, 218. Constant temperature cold junction, 71. Contact-pen recorders, 88. “Critical” points of steel, 94.

_Daniell_, expansion pyrometers, 2, 214. _Darling_, automatic compensator, 68. — and _Grace_, liquid element thermocouples, 43. _Day_, extension of gas scale, 15.

Electromotive force (E.M.F.) developed by junctions, 31. — — measurement of, 62. Expansion pyrometers, 214.

_Fahrenheit_, temperature scale, 7. _Féry_, lens pyrometer, 142. — mirror pyrometer, 143. — optical pyrometer, 174. — spiral pyrometer, 150. Fixed points for calibration of pyrometers, 16, 17. _Foster_, base-metal pyrometer, 39. — fixed-focus pyrometer, 152. — recorder, 82. Furnace, electric tube, 95. — temperatures, control of, by pyrometers, 87. Fusible metals, 209. — pastes, 210. Fusion pyrometers, 204.

Gas pyrometers, 219. — scale of temperatures, 11. — thermometer, constant volume, 11.

_Hadfield_, effect of temperature on hardness of steel, 4. _Harris_, indicator, 121. _Holborn-Kurlbaum_, optical pyrometer, 181. _Holden-d’Arsonval_, galvanometer, 45. _Holman_, formula for thermal junctions, 62. _Howe_, colour-temperature table, 167.

Indicators, for radiation pyrometers, 156. — for resistance pyrometers, 118-124. — special range, 71. — standardizing of, 54, 108, 157. — for thermo-electric pyrometers, 45-53. Installations of resistance pyrometers, 130. — of thermo-electric pyrometers, 89.

_Joly_, meldometer, 220.

_Kelvin_, thermodynamic scale of temperature, 9. _Kowalke_, base-metal couples, 30.

_Lambert_, anti-vibration stand for galvanometers, 47. _Le Chatelier_, optical pyrometer, 177. — thermo-electric pyrometer, 5, 22. _Leeds-Northrup_, indicator, 122. — recorders, 85, 127. — resistance pyrometers, 115. Liquid element thermocouples, 43. _Lovibond_, optical pyrometer, 186. Low temperatures, measurement of, 97, 132.

Meldometer, 220. Mercury thermometer, limits of, 1. _Mesuré and Nouel_, optical pyrometer, 187. _Morse_, optical pyrometer, 182.

National Physical Laboratory, scale of temperatures, 17. _Newton_, researches on high temperatures, 2. _Northrup_, molten tin pyrometer, 216. — pyrovolter, 74.

Optical pyrometers, 167. — — management of, 192. — — special uses of, 193.

_Paul_, base-metal pyrometer, 39. — compensator, 70. — radiation pyrometer, 155. — recorder, 83. — uni-pivot indicator, 49. _Peake_, compensated leads, 67. Pivoted galvanometers, 49. _Planck_, modifications of Wien’s formula, 171. Platinum scale of temperatures, 106. Pneumatic pyrometers, 217. Potentiometer indicators, 73. — method for measurement of E.M.F., 63. _Prinsep_, gas pyrometer, 3. Protecting sheaths for pyrometers, 34. Pyrometer, definition of, 1. Pyromike, 189.

Radiation pyrometers, 134. — — calibration of, 157. — — indicators for, 156. — — management of, 161. — — recorders for, 161. — — special uses of, 164. _Rasch_, luminosity formula, 168. Recalescence of steel, 4, 94. Recorders for radiation pyrometers, 161. — — resistance pyrometers, 124. — — thermo-electric pyrometers, 75. Resistance, measurement of, 102. — of platinum, 105. Resistance pyrometers, 101. — — indicators for, 118. — — management of, 130. — — recorders for, 124. — — special uses of, 132. — pyrometry, terms used in, 111. _Roberts-Austen_, recorder, 76.

Salts, melting points of, 204. _Seebeck_, discovery of thermo-electricity, 3, 20. _Seger_, pyramids or “cones,” 205. _Siemens_, calorimetric or “water” pyrometer, 201. — indicator for resistance pyrometer, 118. — — — thermo-electric pyrometer, 48. — optical pyrometer, 182. — recorder, 81. — resistance pyrometer, 114. Specific heat of nickel, 197. Standardizing of indicators, 54, 108, 157. Standards of temperature, 9. Steam, measurement of temperature of, 98. _Stefan-Boltzmann_, fourth-power law, 139. _Stone_, pyrometer, 209. Surface temperatures, measurement of, 97. Suspended-coil galvanometers, 45-48.

Temperature differences, measurement of, 99. — scales, 7-9. — fixed points of, 16-17. Thermal junctions, changes in, 29. — — choice of metals for, 21. — — E.M.F., developed by, 31. — — methods of joining, 26. — — used in pyrometers, 27. Thermo-electric circuits, 22. — pyrometers, 20. — — calibration curves for, 59. — — for surface temperatures, 97. Thermo-electric circuits, indicators for, 45-53. — — management of, 91. — — practical forms of, 32. — — recorders for, 75-87. — — standardization of, 54. Thermodynamic scale of temperatures, 9. Thermometer, constant volume gas, 11. — mercury, 1. Thermophones, 220. Thread recorder, 78.

_Uehling_, pyrometer, 218. Uni-pivot galvanometer, 49.

Vapour-pressure pyrometers, 217.

_Wanner_, optical pyrometer, 178. Water-cooled cold junction, 33. Water equivalent of calorimeter, 200. “Water” pyrometer, 201. Water-jet pyrometer, 217. _Watkin_, heat recorder, 207. Wedge pyrometer, 190. _Wedgwood_, pyrometer, 211. — test-pieces, 205. _Wheatstone_ bridge for measuring resistance, 104-114. _Whipple_, indicator, 120. _Whipple-Féry_, pyrometer, 154. _Wiborgh_, gas pyrometer, 219. — thermophones, 220. _Wien_, luminosity law, 171.

PRINTED IN GREAT BRITAIN BY NEILL AND CO., LTD., EDINBURGH.