White — Melting Point Methods at High Temperatures. 479 



mainly a question of lag, but is often of considerable practical 

 importance. 



Where a single element is used the furnace is regulated 

 for some time before the melting begins till a rate is assured 

 which will carry it through. This takes time, and also requires 

 judgment and experience, since the behavior of the furnace, 

 when left to itself, will depend not only on its individual 

 peculiarities just then, but on the conditions obtaining for 

 a long while previous. With the control element, the observer 

 is in command of his apparatus from first to last, and is 

 relieved from much uncertainty as to its condition. Again, 

 curves extending over long temperature ranges, with the single 

 element, must be done in sections. The furnace rate keeps 

 falling off, till it becomes necessary to stop, cool dowm a little, 

 and then start again. This not only is tedious, but is a great 

 disadvantage with substances whose condition depends on their 

 previous history and is not wholly a function of their tempera- 

 ture at the instant. In one case we found a very important 

 effect which would either have greatly confused our interpreta- 

 tions or would- have been missed altogether if our curve had 

 not been treated as a unit. 



If meltings are carried on, as recommended in the preceding 

 paper, with a constant temperature difference between furnace 

 and charge, the control element is of course essential. 



As actually used, the control element is merely inclosed in a 

 porcelain tube — completely in ordinary cases, projecting 

 (bare) beyond its open end where a specially close reading of 

 the furnace temperature is wanted. Below about 1000° it can 

 be arranged to be read singly, and also differentially in opposi- 

 tion to the charge element, so as to give directly the tempera- 

 ture difference of furnace and charge. At higher temperatures 

 this differential reading, on account of the tendency to leak- 

 age, requires special arrangements which are ordinarily some- 

 what inconvenient. The difference is then best obtained by 

 subtracting the separate readings, allowing, of course, for the 

 change of furnace temperature which usually occurs between 

 the taking of the two. 



IT. The thermoelements. — Thermoelements of platin-rhodium 

 are always used at high temperatures, on account of their 

 superior constancy. For insulators to use with these, the 

 Konigliche Porzellan Manufactur makes tubes of so-called 

 Marquardtmasse. This material, when unglazed, bends slowly 

 if heated for some time in a horizontal position above 1500 6 , 

 but when used vertically, as in an electric furnace, it is suf- 

 ficiently refractory up to 1600°. It shows traces of conductiv- 

 ity as low as 1200°, but in our comparisons of thermoelements 

 at various temperatures no error due to this conductivity has 



