RECENT ADVANCES IN HIGH TEMPERA- 

 TURE MEASUREMENT 



By J. A. HARKER, D.Sc, F.R.S. 



The National Physical Laboratory, Teddington 



Such rapid strides in the science of pyrometry have been made 

 during the past decade that it may be of interest to review recent 

 progress, particularly of the more practical applications. In 

 order adequately to place the subject of recent advances before 

 the non-specialist reader some elementary explanations as to the 

 principles involved are necessary. The most familiar type of 

 temperature measurer is the well-known ordinary thermometer, 

 in which the general property of the expansion of a body by 

 heat is utilised to obtain a scale of temperature. A bulb forming 

 the containing reservoir is blown upon the end of a fine glass 

 capillary tube of uniform cross-section. A liquid such as 

 mercury or alcohol, having a much greater expansion than that 

 of glass, is filled into the bulb, and the changes in volume of 

 the liquid with temperature are indicated by its rise and fall in 

 the capillary tube. If it be desired to compare the indications 

 of the thermoscope thus obtained with those of other observers, 

 it is necessary, in order to convert it into a thermometer, to 

 adopt some definite " scale," preferably as near as possible the 

 same for instruments of all kinds. For this purpose therefore 

 at least two fixed points must be selected. The two universally 

 chosen are the temperature of melting ice and the boiling-point 

 of water under one atmosphere pressure. 



On the Fahrenheit Scale of temperature in common use in 

 this country, the ice-point is arbitrarily called 32° and the steam- 

 point 212'', the interval between them being divided into 180 

 equal parts called degrees Fahrenheit. Similarly on the Centi- 

 grade Scale which is almost universally used for scientific and 

 now very largely for technical work, the ice-point is called 0° and 

 the steam-point 100°, the Centigrade degree being the jJoth part 

 of this interval. Over this small range all substances, both 



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