362 DR. J. A. BARKER ON THE HIGH-TEMPERATURE STANDARDS 



capillary tubes used to cover them shall anywhere touch the furnace wall. As 

 previous experience with gas and platinum thermometers, whose walls were of 

 porcelain, had shown how very much more slowly the transfer of heat toolc place 

 through this material than through metal or glass, even when surrounded by a stirred 

 liquid, it was judged preferable to make a comparatively small number of high- 

 temperature experiments, in which great constancy of temperature was attained for 

 some time previous to and during the observations, rather than to attempt to obtain 

 mean values from more extended series under less perfect conditions. With this 

 object, a Callendar recorder was connected to a second platinum thermometer placed 

 in the furnace, and, during the adjustment of the temperature and the comparisons, 

 records from this instrument were taken on an open scale. The use of the recorder 

 greatly shortened the time necessary for the establishment of a steady temperature 

 by guiding the observer as to the manipulation of the resistances in the heating 

 circuit. In addition to the large set of resistances in the heating circuit, a set of coils 

 of ()! ohm each was placed close to the recorder, and it was found that a change of 

 one step on this set made all the difference between a steady state and a gradual rise 

 or fall in the temperature of the furnace when equilibrium had been nearly established. 

 When desired, it was quite easy to keep the furnace temperature constant to about a 

 fifth of a degree for half-an-hour at a time, at temperatures as high as 1000 C., but 

 in most of the experiments the temperature was intentionally allowed to rise very 

 slowly. Without these precautions, comparisons between instruments of such widely 

 differing ''lag" as a bare thermojunction and a gas thermometer with porcelain 

 reservoir, whose walls were 2 millims. in thickness, would have undoubtedly been 

 liable to serious error. 



XVII. Furnace Correction. 



In order to investigate the distribution of temperature throughout the space filled 

 by the gas thermometer bulb, which was about 130 millims. in length, a pair of 

 thermojunctions, quite independent of the standard, were arranged so as to measure 

 the temperature difference between the centre of the furnace and points further out, 

 and so obtain a correction to be applied to the readings of the gas thermometer, to 

 reduce its indications to what it would have registered had the whole of the bulb 

 been at the same temperature as the middle point. 



XVIII. Exploration of Furnace. 



For this purpose a thin wire junction of platinum with platinum iridium was chosen 

 on account of its great sensitiveness at high temperatures. This was made up to 

 work differentially, and was composed of a piece of platinum iridium between two 

 pieces of platinum, thus forming two junctions, which were both placed in the hot 

 space, entering the furnace from opposite ends. The wires were stiffened by threading 



