PYROMETRY
CHAPTER I
INTRODUCTION
The term “pyrometer”—formerly applied to instruments designed to measure the expansion of solids—is now used to describe any device for determining temperatures beyond the upper limit of a mercury thermometer. This limit, in the common form, is the boiling point of mercury: 357° C. or 672° F. By leaving the bore of the tube full of nitrogen or carbon dioxide prior to sealing, the pressure exerted by the enclosed gas when the mercury expands prevents boiling; and with a strong bulb of hard glass the readings may be extended to 550° C. or 1020° F. Above this temperature the hardest glass is distorted by the high internal pressure, but, by substituting silica for glass, readings as high as 700° C. or 1290° F. may be secured. Whilst such thermometers are useful in laboratory processes they are too fragile for workshop use; and if made of the length necessary in many cases in which the temperature of furnaces is sought, the cost would be as great as that of more durable and convenient appliances. No other instrument, however, is so simple to read as the thermometer; and for this reason it is used whenever the conditions are favourable. The latest proposal in this direction is due to Northrup, who has constructed a thermometer containing tin enclosed in a graphite envelope, which is capable of reading up to 1500° C. or higher. This instrument is described on [page 216].
The origin and development of the science of pyrometry furnish a notable example of the value of the application of scientific principles to industry. Sir Isaac Newton was the first to attempt to measure the temperature of a fire by observing the time taken to cool by a bar of iron withdrawn from the fire; but, although Newton’s results were published in 1701, it was not until 1782 that a practical instrument for measuring high temperatures was designed. In that year Josiah Wedgwood, the famous potter, introduced an instrument based on the progressive contraction undergone by clay when baked at increasing temperatures, which he used in controlling his furnaces, finding it much more reliable than the eye of the most experienced workman. This apparatus ([described on page 211]) remained without a serious rival for forty years, and its use has not yet been entirely abandoned.
The next step in advance was the introduction of the expansion pyrometer by John Daniell in 1822. The elongation of a platinum rod, encased in plumbago, was made to operate a magnifying device, which moved a pointer over a scale divided so as to read temperatures directly. Although inaccurate as compared with modern instruments, this pyrometer was the first to give a continuous reading, and required no personal attention. The expansion pyrometer—with different expanding substances—is still used to a limited extent.
The year 1822 was also marked by Seebeck’s discovery of thermo-electricity. The generation of a current of electricity by a heated junction of two metals, increasing with the temperature, appeared to afford a simple and satisfactory basis for a pyrometer, and Becquerel constructed an instrument on these lines in 1826. Pouillet and others also endeavoured to measure temperatures by the thermo-electric method, but partly owing to the use of unsuitable junctions, and partly to the lack of reliable galvanometers, these workers failed to obtain concordant results. The method was for all practical purposes abandoned until 1886, when its revival in reliable form led to the enormous extension of the use of pyrometers witnessed during recent years.
In 1828 Prinsep initiated the use of gas pyrometers, and enclosed the gas in a gold bulb. Later workers used porcelain bulbs, on account of greater infusibility, but modern research has shown that porcelain is quite unsuitable for accurate measurements, being porous to certain gases at high temperatures, even when glazed. Gas pyrometers are of little use industrially, but are now used as standards for the calibration of other pyrometers, the bulb being made of an alloy of platinum and rhodium.