Constantan wire tables

Quite recently Eucken and Schwers (84) have made some measurements with the vacuum calorimeter, which were planned particularly to test the TMaw of Debye (cf. following chapter) in view of this object the highest accuracy was wanted at the lowest temperatures (in thermodynamical calculations the specific heats are of subordinate importance in this region because of their smallness). The method used was essentially that described above the heating was done, however, with a constantan wire, while the measurements of temperature were made by means of a lead wire, the resistance curve of this material at very low temperatures being of a more suitable form than is the case with platinum. From the measurements of Kamerlingh-Onnes and Clay the following table was calculated by the above-mentioned authors, and was related to the sample of lead used by them by correction with the aid of the a-rule (p. 36). 

C. For this purpose, a silver-plated table, cooled by an ultracryostat, was used. The thin-walled tubes were fixed onto this table. The crystals were pushed into these tubes with a thin constantan wire, and the tubes were then sealed with a hot tungsten wire. 

Some electric tube furnaces incorporate a p3n ometer, which is not required in any of the experiments described in this manual, though Experiments 13 and 14 can be run by using an electric tube furnace at controlled temperature instead of an oil bath if so desired. A pyrometer may easily be improvised with iron and constantan wires twisted together and spot-welded to provide the junctions. The hot junction is placed against the outside of the combustion tube in the center of the heated portion and bound in position by asbestos tape, which also serves as insulation. The cold junction is kept in ice and water, and the electromotive force is measured with a millivoltmeter. With the cold junction at 0°C and the hot junction at 200°C, the electromotive force of the iron-constantan couple is given in tables as 10.77 millivolts. For accurate work the couple used should be calibrated and to assure uniform temperature distribution and electrical shielding, the combustion-tube and thermocouple wires should be encased in a tubular metal shield that fits inside the furnace and is groimded. 

If you look at a thermocouple calibration table, you will see that it has a reference junction at 0°C. This reference point is used because of an interesting complication that arises when a thermocouple is hooked up to a voltmeter. To explain this phenomenon, first look at one specific type of thermocouple, a type T (cop-per-constantan design). Also, assume that the wires in the voltmeter are all copper. Once the thermocouple is hooked to the voltmeter, we end up having a total of three junctions (see Fig. 2.31) ... 

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