170 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 2 



in Table 10 can be computed from the difference of the values of S. 

 Example: Copper versus platinum, 6.5; constantan versus plati- 

 num, — 35; copper versus constantan. 6.5 — (—35) = 41.5 / aV/°C. 



Because of the variation of the sensitivity S with temperature, the 

 sequence of the materials in Table 10 varies with temperature, too. 

 Calibration curves and tables for a number of standardized thermo- 

 couple combinations are available from the National Bureau of 

 Standards and from the Leeds and Northrup Company in Phila- 

 delphia. For thermoelements made from nonstandard materials, 

 empirical calibration is required. 



The more common types of thermocouples are described in Table 

 1 1 . For detailed information concerning the properties and specifi- 

 cation for standardization of the metals used in these thermoele- 

 ments, see the monograph on thermoelectric thermometry by Dike. 1 

 For general information on thermoelectric thermometry, see also 

 Roeser. 2 



The wires from which thermoelements are to be made should be annealed 

 at a temperature higher than that at which the thermoelement is to be used. 

 It is important that the wire is homogeneous, since otherwise a temperature 

 gradient along the wire will cause parasitic thermal emfs (Becquerel effect). 

 The wire can be tested for homogeneity by connecting both ends to a galvanom- 

 eter and moving a bunsen burner along the wire; the galvanometer should 

 not show any deflection. The contamination of platinum or platinum-rhodium 

 wires can sometimes be removed by heating the wire for a few minutes to about 

 1300°C to oxidize the impurities. Oxides can be removed from the surface 

 with a molten borax bead flowing down the wire. 



The wires can be joined together by soldering or welding. Platinum thermo- 

 elements are usually welded (without twisting the end) in an oxygen-hydrogen 

 flame or in a carbon arc without the use of a flux. Copper-constantan and iron- 

 constantan thermoelements can be twisted with moderate strain and silver- 

 soldered. Some authors recommend the use of ammonium chloride or rosin 

 flux. Borax or fluorspar is recommended as a flux for chromel-alumel thermo- 

 elements. Thin wires can be welded by discharging a capacitor of several 

 microfarads, charged to a voltage of 20 to 100 volts, through the junction. Soft 

 solder (tin-lead) and low-melting alloys, such as Woods metals, are satisfactory 

 at lower temperatures. See F. Kerkhof, Arch. tech. Messen, J 2404-1, October, 

 1940. Thermoelements must be protected from mechanical injuries, chemical 

 contamination, and electrical disturbances (leakage, a-c pickup, galvanic emf 

 in electrolytes). Platinum-platinum rhodium thermocouples are particularly 

 susceptible to damage if exposed to vapors of other metals, or to hydrogen, 

 sulfur, and carbon monoxide. Thin wires are more susceptible to contamina- 

 tion than thick wires. Protection in metal or ceramic tubes is required in most 



1 P. H. Dike, "Thermoelectric Thermometry," Leeds and Northrup Com- 

 pany, Philadelphia, 1954. 



2 W. F. Roeser, in "Temperature," pp. 180ff, Reinhold Publishing Corpora- 

 tion, New York, 1941. 



