301 
Thermoelectric Pyrometer—Numerous materials have been used 
around the laboratory for thermo-couples, but the cheapest and at the same 
time the most reliable is the copper-constantan. The latter metal is known 
in this country as Advance, or Ia In. This couple can be used up to 
about 900° C. Av extended investigation of this thermoelement has been 
carried out by White,’ who recommends it as a precision thermometer. For 
temperatures between 300° C. and 1600° C. platinum and some alloy of 
platinums must be used. 
The choice of a couple depends entirely upon the conditions under 
which it is to be used. For high temperatures the platinum couple 
(Pt——Pt+10%Rh) is perhaps the only one that is used with success, but 
for low temperatures, say up to 1000° C., a number of alloys are used in in- 
dustrial processes with good success. For low temperatures it is necessary 
to choose metals that will produce a higher P.D. than that used at high 
temperatures. For temperatures below 100° C., the couple may be cali- 
brated by direct comparison with mercury thermometer, but for high tem- 
peratures fixed points are necessary. 
The method of measuring the P.D. depends upon the accuracy required. 
For precise work the cold junction should always be kept at constant tem- 
perature (generally melting ice) and the P.D. should be measured on a 
potentiometer, using a standard cell. For work when great precision is not 
necessary, a d’Arsonval galvanometer or even a sensitive millivoltmeter is 
sufficiently accurate. In industrial practice the outfit must be as portable 
and compact as possible so that a direct reading instrument is generally 
used, which is substantially a millivoltmeter calibrated to read direct in 
temperature °C. or °F. The cold junction in such cases is generally main- 
tained at 25° C. or 75° F., and the instrument is calibrated to be correct at 
that temperature. .Any slight variation will not cause.a great error, but 
all approximate correction can always be made by adding to the indicated 
temperature the difference between the temperature of the cold junction 
and 25°, when the former exceeds 25°, and subtracting the difference when 
it is less than 25°. Correction can also be made by means of an automatic 
4 Bulletin Bureau of Standards, Vol. 6, p. 149. 
5 Phys. Rev., Aug., 1910, p- 135s 
