295 
men who are handling this problem in the industries are not sufficiently 
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 surprising 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, ete. 
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 high 
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.—Yhis instrument utilizes the variation of 
the electromotive force with temperature, developed at the junction of two 
dissimilar metals. This pyrometer may be used for temperatures up tw 
1600° C. when the thermo-couple consists of wires of platinum and platinun- 
rhodium or iridium. 
Radiation Pyrometer.—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 Pyrometer.—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. 
