HIGH TEMPERATURE MEASUREMENT 503 



action of furnace gases. Vapours of metals, such as iron, which 

 in an oxidising atmosphere has an appreciable tension at i ,000° C, 

 rapidly destroy the platinum metals and should be particularly 

 guarded against. 



Recent improvements in the apparatus used for resistance 

 measurements, and in particular the introduction for use in 

 resistance coils of metals having practically no temperature 

 coefficient, have rendered possible the design of portable com- 

 mercial forms of platinum pyrometer outfit and capable of 

 giving high accuracy. One of these forms is made direct 

 reading on the gas-scale, if required; when arranged for use 

 up to a maximum temperature of 1,200° it can be set to |^ C. 

 Such pyrometers are eminently suitable for the heat treatment 

 of steel over the range 700° to 900" C, where, in some kinds of 

 commercial practice, a difference of 5° C. in the hardening 

 temperature makes a quite perceptible difference in the nature 

 of the product obtained. 



Recently a new form of platinum thermometer has been 

 introduced, in which the wire, instead of being wound on to 

 a mica cross, is wrapped upon a tube of clear, fused silica, a 

 second silica tube being shrunk over this for mechanical pro- 

 tection. Thermometers of this type may be constructed so 

 that their lag is very small — a matter of importance in some 

 cases. It has been found, however, that at high temperatures 

 fused silica devitrifies slowly, reverting to its crystalline form 

 of tridymite, and that it is therefore not desirable to submit 

 any pyrometers in which this material is employed to tempera- 

 tures above about 900' C. except for short periods. 



Platinum thermometers have the advantage that they can 

 easily be arranged to give a continuous record, and for purposes 

 where automatic temperature registration is desirable over long 

 periods they are extensively used. For records of the tempera- 

 ture of the hot blast for iron furnaces, and for flue and steam 

 temperatures, they are probably the most suitable instruments 

 to employ. For use in scientific work as in practical standards, 

 they present the great advantage over thermocouples that the 

 same instrument may be used, if required, for a range of from 

 0° to 1,100° C. with approximately constant sensitiveness; also 

 that it is easy to design instruments, whose size and shape, 

 resistance and sensitiveness are best adapted to the particular 

 purpose in view, 



