502 SCIENCE PROGRESS 



use it successfully even at moderate temperatures is extremely 

 difficult. Much labour has therefore been spent on the evolu- 

 tion of suitable appliances to serve as " working " rather than 

 " primary " standards. Of these the most successful and easily 

 applicable for moderate ranges are the electrical methods of tem- 

 perature measurement, the first of these being the method of the 

 electrical resistance thermometer. This depends for its applica- 

 tion on the fact that the resistance of most conductors made of 

 pure metals changes rapidly with the change of temperature. 

 For the higher temperatures the refractory character, ductility 

 and general properties of platinum first used by Siemens have 

 caused it to be specially selected for this purpose and in most 

 resistance methods this metal is now used. 



The platinum thermometer has the great advantage that, 

 provided the wire used for the " bulb " be comparatively pure, 

 standardisation ,at three points only is required to establish a 

 temperature scale serving for measurements up to i,ooo° C. If 

 the symbol pt be employed to denote any temperature on the 

 Centigrade platinum scale, and R, Rjoo and Rq are the ob- 

 served resistances of the thermometer at the temperatures /»/, 

 loo'' and o° respectively, 



,, R—Ro 



pt= j^ 7j- X 100. 



To reduce temperatures on the platinum-scale to the gas-scale 

 the relation between T and // must be known, Callendar 

 showed that up to 600° C. their difference could be expressed 

 by the equation 



|_\ioo/ 100 J 



the value of S for most pure wires being approximately r5. 

 Following his suggestion platinum thermometers are now almost 

 universally standardised by determination of their resistance in 

 ice, steam and sulphur vapour. 



Platinum thermometers having a protector tube of glazed 

 porcelain may be employed up to 1,100° or 1,200° C. for inter- 

 mittent, and to 800° or 900° C. for continuous work without 

 serious changes taking place in the wire, provided the instru- 

 ment has before use been subjected to a thorough annealing 

 process. Care should also be taken that the protecting sheath 

 be quite gas-tight, to preserve the platinum from the reducing 



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