300 
of thermodynamics. Consequently the “temperature” which occurs in the 
equations is the absolute thermodynamic temperature. 
The standardization of pyrometers is generally made by means of cer- 
tain fixed points, such as the fusion of platinum, palladium, gold, ete, 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 
up 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 seale. 
It is seen from the above that high temperature measurements may 
be made in terms of the thermodynamic scale, but that the actual precision 
1s entirely subordinate to that of the various intermediate steps, which 
lead from the perfect gas thermometer to the radiation pyrometer. 
APPARATUS. 
Blectric-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 
used for high temperatures (up to 1000° C.) the platinum coil is generally 
wound over a mica frame and jnclosed in an infusible porcelain stem. 
