HIGH TEMPERATURE MEASUREMENT 505 



a pointer moving over a graduated scale or by the familiar 

 "light-spot" method of the physical laboratory invented by 

 Lord Kelvin. Owing to the fact that the electromotive force 

 given by these couples is comparatively small it was not found 

 practicable till the last few years to make a good portable 

 galvanometer on which the temperature readings could be 

 taken. Now, however, there are on the market several types 

 of commercial instrument which are amply sensitive without the 

 use of the light-spot and are free from zero-drift. It is possible 

 with care with such an instrument of the better class to measure 

 moderate temperatures to about i°C., if the temperature of the 

 " cold junctions " — that is, the point where the two wires of the 

 couple itself are joined to the copper connecting-leads — be known 

 to a sufficient degree of accuracy. Unless the temperature co- 

 efficient of the galvanometer employed be sufficiently small and 

 its resistance sufficiently high, resistance changes in the circuit 

 also affect the result, generally in a somewhat uncertain manner ; 

 hence for the highest class of work the directly deflected 

 galvanometer is replaced by a suitable potentiometer, in which 

 part or the whole of the electromotive force generated by the 

 couple is balanced. On such an instrument, if correctly designed, 

 it is possible with ease to follow temperature changes to '05° C. 

 during the freezing of a pure silver crucible in an electric 

 furnace or to measure the melting-point of platinum on the 

 thermo-electric scale to within i°C. 



One of the most important recent improvements in thermo- 

 electric pyrometry has been due to the fact that two firms have 

 succeeded in producing large homogeneous ingots of platinum 

 and of the lo-per-cent. alloys used for thermocouple wires 

 in a state of high commercial purity, and that thus it is 

 possible to substitute for any couples others giving the same 

 constants. This saves much labour in the recalibration of 

 instruments. 



Base metal couples of various kinds have been introduced for 

 work at lower temperatures. Thus for temperatures to 800° C. 

 the alloy of copper and nickel, known as eureka or constantan, 

 may be used against a wire of soft copper or iron. The sen- 

 sitiveness of the latter combination is roughly 50 microvolts 

 per degree C, increasing slightly with rise of temperature. 



During the past decade very great progress has been made 

 in the development of some entirely new methods of temperature 



33 



