298 



men who are liaiiclling this problem in the industries are not sutiiciently 

 trained to appreciate the limitations of its practical application. There- 

 fore the engineer or chemist must be trained along this line if he expects 

 to do the most efficient work. High temperatures were, until quite re- 

 cently, estimated by the trained eye of a workman, and while they acquired 

 with practice a surpL'ising accuracy, such a procedure is entirely inadequate 

 for present day requirements. Moreover, the observer's estimate is influ- 

 enced manifestly by a number of circumstances, such as the amount of 

 light in the room, fatigue of the eye, physical condition of the observer, etc. 

 The greatest disadvantage is that a skilled workman in Pittsburg can not 

 gain anything from the experience of a workman in Birmingham. In times 

 past numerous methods have been devised and used for temperature estima- 

 tion and temperature control, but the temperature scales used were so dis- 

 cordant that about six years ago the Bureau of Standards- made a thorough 

 investigation of the most available methods. 



There are today four precise laboratory methods for measuring higli 

 temperatures, each of which is the basis of an industrial pyrometer: 



Electric-resistance Pyrometer. — In this pyrometer use is made of the 

 variation of the electric resistance of metals with change of temperature. 

 Since resistance can be measured with extreme precision this method per- 

 mits of very precise measurements of temperature up to 1000° C. 



Thermoelectric Pyrometer. — This instrument utilizes the variation of 

 the electromotive force with temperature, developed at the junction of two 

 dissimilar metals. This pyrometer may be used for tem]>eracures up tn 

 10(X)° C. when tlie thermo-couple consists of wires of platinum ;!nd platinum- 

 rhodium or iridium. 



Radiation Pi/rovieter. — In this type of pyrometer the total radiation 

 from hot bodies is taken as a measure of their temperature. This instru- 

 ment requires a device for determining very small changes in temperature, 

 and does not admit of very great accuracy, but is very convenient for very 

 high temperatures. 



Optical Pj/romcter. — In the case of pyrometers of this class tempera- 

 ture estimation is made by means of a photometric comparison, for a par- 

 ticular wave length, between the radiation from some standard lamp and 

 the radiation emitted from the body under observation. This is a very 

 precise method and is available for the highest known temperature. 



Bulletin Bureau of Standards, Vol. 1, p. 189. 



