Pyrometry: A Practical Treatise on the Measurement of High Temperatures. Charles R. Darling

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СКАЧАТЬ the metals mechanically. When measuring high temperatures with this instrument, a considerable pressure, amounting to 1 atmosphere for every increase of 273 degrees above the ice point, is requisite to prevent expansion of the nitrogen; and this pressure tends to distort the bulb and so to falsify the indications. This trouble has been overcome by Day, who surrounded the bulb by a second larger bulb, and forced air or nitrogen into the intervening space until the pressure on the exterior of the thermometer bulb was equal to that prevailing in the interior. Even then it was not found possible to secure higher readings than 1550° C., as the bulb commenced to alter in shape owing to the softening of the material. This temperature represents the highest yet measured on the gas scale; but by using a more refractory material, such as fused zirconia, it may be found possible to extend this range to 2000° C. or more. Experiments in this direction are very desirable, in order that high-reading pyrometers may be checked directly against the gas scale.

      Fixed Points for Calibration of Pyrometers.—It is evident that the gas thermometer is totally unsuited for use in workshops or laboratories when a rapid determination of a high temperature is required. Its function is to establish fixed points or temperature standards, by means of which other instruments, more convenient to use, may be graduated so as to agree with each other and with the gas scale itself. The temperature scales of all modern pyrometers are thus derived, directly or indirectly, from the gas thermometer. In the table on next page, a number of fixed points, determined by various observers, is given; the error, even at the highest temperatures, probably not exceeding ±2° C.

      In preparing the temperature scale of a pyrometer for practical use, the instrument is subjected successively to a number of the temperatures indicated in the table, and in this manner several fixed points are established on its scale. The space between these points is then suitably subdivided to represent intermediate temperatures.

      Table of Fixed Points.

Substance.	Physical Condition.	Deg.	Deg.
		Cent.	Fahr.
Water (ice)	At Melting Point	0	32
Water	” Boiling ”	100	212
Aniline	” ” ”	184	363
Naphthalene	” ” ”	218	424
Tin	” Melting ”	232	449
Lead	” ” ”	327	620
Zinc	” ” ”	419	786
Sulphur	” Boiling ”	445	833
Antimony	” Melting ”	631	1167
Aluminium	” ” ”	657	1214
Common Salt	” ” ”	800	1472
Silver (in air)	” ” ”	955	1751
Silver (free from oxygen)	” ” ”	962	1763
Gold	” ” ”	1064	1947
Copper (in air)	” ” ”	1064	1947
Copper (Graphite covered)	” ” ”	1084	1983
Iron (pure)	” ” ”	1520	2768
Palladium	” ” ”	1549	2820
Platinum	” ” ”	1755	3190

      It is necessary to point out that the figures given in the table refer only to pure substances, and that relatively small quantities of impurities may give rise to serious errors. The methods by which the physical condition to which the temperatures refer may be realised in practice will be described in the succeeding chapter.

      National Physical Laboratory Scale.—Exact agreement with regard to fixed points has not yet been arrived at in different countries, and an effort to co-ordinate the work of the National Physical Laboratory, the United States Bureau of Standards, and the Reichsanstalt, with a view to the formation of an international scale, was interrupted by the war. In 1916 the National Physical Laboratory adopted a set of fixed points on the Centigrade thermodynamic scale, in conformity with which all British pyrometers have since been standardised. It will be seen that the figures differ very slightly from those given in the previous table, which represent the average results of separate determinations in different countries.

      National Physical Laboratory Scale (1916)

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Substance.	Physical Condition.	Deg.	Deg.
		Cent.	Fahr.
Water (ice)	At Melting Point	0	32
Water	” Boiling ” (760 mm.)	100	212
Naphthalene	” ” ” ”	217·9	424
Benzophenone	” ” ” ”	305·9	582
Zinc	At Melting Point	419·4	787
Antimony	” ” ”	630	1166
Common Salt	” ” ”	801