A multirange potentiometer and its application to the measurement of small temperature differences
H.B. Brooks, A.W. Spinks · Bureau of Standards Journal of Research · 1932
In determining, as a criterion of purity, the boiling range or the freezing range of a liquid which is being purified by fractionation, it is convenient to be able to determine accurately the small difference between the boiling points or the freez- ing points of the two end fractions.The occasional measurement of relatively large temperature differences between two fractions also requires that the total range of measurement be large in comparison with the smallest measurable tem- perature difference.Thermocouples are very suitable for measurements of temper?ture difference.To increase the electromotive force per degree it was decided in a particular case to use a group of 10 copper-constantan thermocouples in series.Even with these 10 couples, the electromotive force corresponding to a difference of 0.001°C.would be only 0.4 /*v, and its measurement to the nearest 0.0001°would require the measurement of the electromotive force to 0.04 juv.Measurements accurate to such a small quantity require not only that the potentiometer used shall be extraordinarily free from parasitic electromotive force, but also that provision should be made for readily detecting and eliminating any such intruding electro- motive force in the galvanometer circuit.The relatively little-used "second method" of Poggendorff, as first realized in practical form by Lindeck and Rothe, appeared to offer the most desirable basis for the design of a multirange potentiometer suitable for the above purposes.The potentiometer which will be described has 6 ranges, and when used with the group of 10 thermocouples provides 6 ranges of temperature difference; namely, from 0°t o 0.1°, 0.2°, 0.5°, 1°, 2°, and 5°, respectively, corresponding in each case to a deflection of the pointer of 100 divisions, readable by estimation to 0.1 division.The potentiometer is readily adaptable to read directly in microvolts or in temperature difference, as may be preferred.For the particular application for which it was designed, it was desired to avoid entirely the use of the curves or tables which are necessary when the readings are in microvolts, and it was therefore arranged to read directly in temperature difference.