Reaction before Complete Equilibrium, 



75 



The Thermometer {Pt.). — In the pyrometers now in use 

 the platinum resistance-coil is inclosed in a glass or porcelain 

 tube, the walls of which are ahout 1 to 2 mm. thick, and the 

 coil is suspended in air. Air, glass, and porcelain being very 

 bad conductors of heat, it assumes the temperature very 

 slowly, especially the last thousandths or ten-thousandths ot 

 a degree. The pyrometer, being a long tube, is never com- 

 pletely immersed in the liquid. Hence the temperature of 

 the air in the tube must necessarily be different from that 

 of the liquid under investigation. 



In these investigations it was absolutely necessary that the 

 thermometer should indicate instantaneously the temperature 

 of the liquid. For this reason the pyrometer was constructed 

 in the way shown in figs. Pt, and Pt,,. In fig. Pt lt the 



Ptu 



platinum wire is wound on a mica plate placed diagonally in 

 the gilt copper case A, the two opposite sides of the case being 

 4 mm. apart. The four copper tubes 1. '2. 3, 4 contain glass 

 tubes in which pieces of gilt manganin wire of equal lengths 

 and of a temperature-coefficient 0*0000035 are fused to short 

 lengths of platinum wire melted into the bottoms of the glass 

 tubes. The four copper tubes have been fixed in the lid of 

 the copper case. After the four short pieces of platinum 

 wire were fixed to the top of the mica plate, two of the wires 

 were fused to the ends of the platinum coil on the mica ; the 

 other two were fused together, and formed the compensating- 

 leads. The copper case containing the platinum coil, a great 



