474 White — Melting Point Methods at High Temperatures. 



Art. XLY. — Melting Point Methods at High Temperatures ; 

 by Walter P. White. 



In an earlier paper of this series*, it has been shown that 

 for melting-point determinations up to 1600° C. the platinum 

 thermoelement, when nsed with a potentiometer (and with 

 suitable precautions against the effects of contamination), is 

 accurate, convenient, and rapid ; the resistance-thermometer 

 is inadequate in range, and ill adapted to the special con- 

 ditions encountered ; the direct reading pyrometer is seriously 

 lacking in sensitiveness and accuracy and has very little advan- 

 tage in convenience where portability is not essential. Further- 

 more, potentiometers and galvanometers now on the market 

 are sufficient for the needs of the work, though special instru- 

 ments can be devised which are both cheaper and more con- 

 venient. As low as 1000° most solid insulators show signs of 

 an electrolytic conduction which increases with the tempera- 

 ture, while at about 1300° the air itself becomes conducting 

 through ionization, but the disastrous effect which the rela- 

 tively high voltage of the furnace current then produces on 

 the delicate electrical temperature measurement can be com- 

 pletely avoided by protecting the measuring system with a 

 metallic equipotential shield. 



The present paper continues the discussion of apparatus 

 and of the varying procedure necessary to determine melting 

 curves according as the main object is the accurate location of 

 heat absorptions, their quantitative measurement, or their mere 

 detection when obscure. 



Part I. The Apparatus. 



I. The furnace. — The furnaces used in this laboratory are 

 nearly always of the type described by Day and Allen live 

 years ago.f For melting-point determination it has proved so 

 satisfactory that further experience has suggested hardly any 

 changes except variations in dimensions to suit different classes 

 of work. Platinum wire of smaller size (l*2 mm ) has been 

 found to answer, which cuts the cost nearly in half, but a 

 specially pure platinum is now nsed in order to prevent con- 

 tamination of the thermoelement by the iridium vapor which 

 comes from the commercial metal. The furnace consists (1) 

 of the coil, which is more effective the nearer it is to the 



* Potentiometer Installation, especially for High Temperature and Thermo- 

 electric Work ; Phys. Rev., xxv, 334, 1907. 



f Temperature Measurements to 1600° C. ; Phys. Rev., xix, 177, 1904. 



