301 



ThcrmoelGctric Pyrometer. — Numerous materials have been used 

 around the hiboratory 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 la la. This couple can be used up to 

 about 900° C. An 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 re<iuired. 

 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'Arsouval 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 2.5° 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 

 an 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 



■• Bulletin Bureau of Standards, Vol. G, p. 140, 

 ' Phys. Rev., Aug., 1910, p. 135, 



