300 



•of thermodynamics. Consequeutly the "temperatiu'e" which occurs in the 

 ^equations is the absolute thermodynamic temperatui-e. 



The standardization of pyrometers is generally made by means of cer- 

 tiiin fixed points, such as the fusion of platinum, palladium, gold, etc., and 

 the ebullition of water, aniline, naphthaline, sulphur, etc., which have been 

 carefully determined by means of the gas thermometer. The platinum 

 thermoelectric pyrometer, on account of its ease of manipulation, conveni- 

 ence, and accuracy, has come into general use for temperature measure- 

 ments between 1200° C, the upper limit of the gas thermometer, and 1600° 

 C. The thermo-couple may be directly compared with the gas thermometer 

 Tjp to 1200° C. but beyond this we must rely on extrapolation up to 1600° C, 

 which is the limit of the thermo-couple. Beyond this range, the scale must 

 depend upon radiation laws which have some theoretical support and can 

 be tested within the range of the gas scale. 



It is seen from the above that high temperature measurements may 

 be made in terms of the thermodynamic scale, but that the actual precision 

 Is entirely subordinate to that of the various intermediate steps, which 

 lead from the perfect gas thermometer to the radiation pyrometer. 



APPARATUS. 



Electric-resistance Pyrometer. — Pyrometers of this type are more or 

 less familiar to persons who have had any experience whatever in Heat 

 or Electrical Measurements' laboratories. To illustrate the application of 

 resistance thermometry, in the laboratory, a number of pure metals such as 

 nickel, iron, silver, and copper may be used for temperatures up to 300° C. 

 and there are several types of cheap, compact, serviceable instruments now 

 on the market. For practical use and calibration the coil of wire used 

 should be inclosed in a tube or stem of some suitable material, such as 

 glass, iron, or porcelain, depending upon the temperature to which it is 

 subjected. This stem should terminate in a head provided with binding 

 post for making connections to lead wires. As the resistance of the, lead 

 wires will vary with the depth of immersion it is necessary to provide com- 

 pensating leads which are put in the adjacent arm of a Wheatstone bridge. 

 For all temperatures, from the lowest obtainable up to 1000° C, and espe- 

 cially for the higher temperatures, platinum" is the most satisfactory. When 

 usod for high temperatures (up to 1000° C.) the jilatinum coil is generally 

 wound over a niica t"r<inie and inclosed in an infusible porcelain stem, 



