172 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 



Extension 

 wires 



applications, but such procedure increases the time lag of the thermoelement 

 and is likely to cause the thermoelement temperature to be different from the 

 temperature to be measured. Metal tubes are usually adequate for the base- 

 metal thermocouples, which are protected from contamination by their oxides, 

 but may be entirely unsatisfactory if furnace gases are present. Sometimes 

 two tubes are used, e.g., a tube of fused silica inside a second tube of metal, 

 silicon carbide, or fire clay. The primary tube of low volatility is intended 

 to be impervious to gases at high temperatvires, and the secondary tube pro- 

 vides resistance to mechanical and thermal shock and corrosion. All ceramic 

 materials tend to become conductive at higher temperatures. For further refer- 

 ence on protection tubes, see P. H. Dike, op. cit., chap. 4F. 



A needle thermoelement for subcutaneous and intramuscular tem- 

 perature measurement is described by Krog. 1 Thermoelements with 

 extremely small heat capacity are frequently used for radiation 

 measurements (see 5- 14a). High-velocity thermocouples for gas- 

 temperature measurements are described by Mullikin. 2 



reference point. The temperature / 2 of the measuring junction 

 P 2 in Fig. (2-1)12 can obviously only be determined if the tempera- 

 ture t x of the reference junctions P 1 and P 3 is constant and known. 



Technical difficulties in keeping the 

 reference junctions at a constant 

 temperature may arise, for instance, 

 if the junction points P 1 and P 3 are 

 in the vicinity of a furnace. It is 

 customary, in such cases, to connect 

 the two wires A and B with two 

 extension wires A' and B' leading 

 to a place t R where the reference 

 temperature is constant, as shown 

 in Fig. (2-1)13. The materials of 

 A' and B' must be chosen so that 

 the thermal emf at the junction 

 points P 1 and P 3 is negligible. Such 

 extension wires (for instance, a chromel-alumel pair) made up as 

 an insulated duplex cable are commercially available for most 

 practical thermocouple combinations. Because of the high price of 

 platinum thermoelements, extension wires for platinum-platinum 

 90-rhodium 10 elements usually consist of copper and a nickel- 

 copper alloy. The thermoelectric emf of these materials versus 

 platinum and platinum rhodium is negligible within a limited 

 temperature range. 



1 J. Krog, Rev. Sci. Instr., 25, 799 (1954). 



2 H. F. Mullikin, in "Temperature," p. 775, Reinhold Publishing Corporation, 

 New York, 1941. 



Fig. (2-1)13. Thermoelement arrange 

 ment with extension wires. 



