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.
Fig. 68.—Wedgwood’s Pyrometer.
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.
