the Roberts-Austen Recording Pyrometer. 61 



produced in the electrical resistance of a coil of platinum win* 

 by a change of temperature, was adapted to industrial use by 

 Sir W. Siemens, and has been greatly improved and widely 

 utilized by Callendar and Griffiths, and by Heycock and 

 Neville. The other system utilizes the electromotive 

 force produced in a circuit of two or more metals when one 

 junction is heated ; its early use is associated with the names 

 of Becquerel (1826) and Tait (1873), while more recently it 

 has been employed by H. Le Chatelier, Barus, and Roberts- 

 Austen. The thermo-couples in general use consist of platinum 

 and platinum alloyed with about 10 per cent, of either 

 iridium or rhodium, the former alloy having been first used by 

 Tait, the latter by Le Chatelier. 



The platinum-resistance pyrometer, or platinum thermo- 

 meter, as it is often called, appears to be capable of affording 

 more accurate results than the thermo-couple especially up 

 to about 600°. This mainly arises from the fact that the 

 measurement of a resistance can be made with a greater 

 degree of accuracy than the measurement of the extremely 

 small E.M.F. produced by the thermo-couple. Moreover, 

 the whole of the coil of wire in the resistance-pyrometer is in 

 the region the temperature of which is to be measured ; hence 

 the indications do not depend, as do those of the thermo- 

 couple, on the absolute uniformity of composition of the 

 platinum wire employed. At higher temperatures, however, 

 the resistance-pyrometer is more seriously affected than is the 

 thermo-couple by the difficulty of securing good insulation, 

 and by the difficulty of obtaining uniformity and constancy 

 of the temperature to be measured ; although the difficulty 

 of insulation has been practically overcome by Callendar' s 

 use of mica supports. The resistance-pyrometer, there- 

 fore, although it may be capable of affording more accurate 

 results than the thermo-couple, is much less convenient for 

 use at high temperatures. The latter, consisting of two thin 

 wires which may be inserted into a small protecting tube of 

 fireclay or porcelain, can be used to measure the temperatures 

 of very small enclosures, and its relatively small lag renders 

 it suitable for obtaining autographic temperature records. 



The present research was begun with the object of modi- 

 fying the Roberts- Austen pyrometer, in order that it might 

 be used to obtain large-scale temperature records having as 

 high a degree of accuracy as possible. It was intended to 

 study the degree of constancy which could be obtained in the 

 indications of the thermo-couple ; to make observations of 

 the melting-points of several metals by means of a thermo- 

 couple; and to calibrate the couple by means of the porcelain 



