242 Tables 216 (continued) and 21 7 



THE INTERNATIONAL TEMPERATURE SCALE 



TABLE 216 (continued). — Recommended Procedure for Calibration 



beyond each end of the thermometer coil. There should be ample opening at 

 the top of the cylindrical and below the conical portion to permit free circula- 

 tion of vapor. The inner surface of the shield should be a poor reflector. The 

 shield may be made of sheet metal, graphite, etc. 



In standardizing a thermometer the sulphur is heated to boiling and the 

 heating so regulated that the condensation line is at least I cm above the top 

 of the insulating material. The thermometer with its radiation shield is inserted 

 in the vapor, and when the line of condensation again reaches its former level 

 simultaneous observations of resistance and barometric pressure are made. In 

 all cases care should be taken to prove that the temperature is independent of 

 vertical displacements of the thermometer and shield. 



5. Silver and Gold 



For standardizing a thermocouple, the metal to be used at its freezing point 

 is contained in a crucible of pure graphite, refractory porcelain, or other ma- 

 terial which will not react with the metal so as to contaminate it to an appre- 

 ciable extent. 



Silver must be protected from access of oxygen while heated. 



The crucible and metal are placed in an electric furnace capable of heating 

 the contents to a uniform temperature. 



The metal is melted and brought to a uniform temperature a few degrees 

 above its melting point, then allowed to cool slowly with the thermocouple 

 immersed in it as described in the next paragraph. 



The thermocouple, mounted in a porcelain tube with porcelain insulators 

 separating the two wires, is immersed in the molten metal through a hole in 

 the center of the crucible cover. The depth of immersion should be such that 

 during the period of freezing the thermocouple can be lowered or raised at least 

 1 cm from its normal position without altering the indicated e.m.f . by as much 

 as 1 microvolt. During freezing, the e.m.f. should remain constant within 

 1 microvolt for a period of at least five minutes. 



As an alternative to displacing the couple, as a means of testing the absence 

 of the influence of external conditions upon the observed temperature, both 

 freezing and melting points may be observed and if these do not differ by more 

 than 2 microvolts, the observed freezing point may be considered satisfactory. 



TABLE 217. — The Standard Platinum Resistance Thermometer 



The diameter of the wire should not be smaller than 0.05 or larger than 

 0.2 mm. 



The platinum wire of the thermometer must be so mounted as to be subject 

 to the minimum of mechanical constraint, so that dimensional changes accom- 

 panying changes of temperature may result in a minimum of mechanical 

 strain being imposed upon the platinum. 



The design of the thermometer should be such that the portion, the resistance 

 of which is measured, shall consist only of platinum, and shall be at the uniform 

 temperature which is to be measured. This may be accomplished by either of 

 the accepted systems of current and potential, or compensating leads. 



After completion, the thermometer should be annealed at a temperature of 

 at least 66o°. 



Smithsonian Tables 



