Stability of Double-Walled Manganin Resistors

The resistance standard described by James L. Thomas [1] was the result of his extensive effort to develop a new standard by systematically investigating every factor affecting the stability of resistance— time, surface effects, temperature, power, pressure— detectable at the time. The result was a unique standard which was used as part of the National Reference Group of resistors beginning in 1931. Ten of them served solely as the U.S. standard of resistance from 1939 until they were supplanted by the quantized Hall effect (QHE) in 1990. They still serve as working standards at the one ohm level and as a vital check on the QHE standard and the scaling used in the NIST resistance calibration service. The International Bureau of Weights and Measures used this standard to maintain the international unit of resistance, and numerous other national standardizing laboratories around the world used it as their primary standard. This is still largely true for laboratories without QHE standards. In the period from 1935 to 1980, Thomas’s standard provided a basis for evaluating the accuracy of ohm determinations, particularly to compare realizations based on calculable inductors with those based on Thompson-Lampard calculable capacitors. Thomas’s standard was commercialized by the Leeds and Northrup Company and Honeywell, and these commercial versions are still used as primary resistance standards by many industrial and commercial standards laboratories, as well as the DOD primary and secondary metrology laboratories. NIST still routinely calibrates about 125 of them annually for domestic users. Thomas’s standard remains the most stable resistor of any available, although two more modern designs are nearly a match in predictability. Much of the research leading to this standard resistor design is described in an earlier paper by Thomas [2]. However, the paper Stability of Double-walled Manganin Resistors [1] is the more popularly known and describes the standard in its final form, after some major modifications in size and connections. In the 1920s, Thomas had taken up the task of improving the long-term stability of wire-wound resistors, which were used to measure the current in absolute determinations. When a resistor is made by winding wire on a spool, parts of the crystalline structure of the wire are stressed past their elastic limit. Thomas developed wire-wound standard resistors that were annealed at high temperature, which released some of the internal strains and reduced the rate of change of resistance with time. Heat-treated manganin wire resistors developed by Thomas incorporated hermetically-sealed, doublewalled enclosures, with the resistance element in thermal contact with the inner wall of the container to improve heat dissipation. These 1 Thomas-type standards (see Fig. 1) proved to be quite stable with time [1,2], and quickly came into favor as the primary reference for maintaining the resistance unit at NBS and at many other national metrology institutes. Work continued on improving the absolute measurements of electrical units and, in 1949, J. L. Thomas, C. L. Peterson, I. L. Cooter, and F. R. Kotter published a new measurement of the absolute ohm [3] using an inductor housed in a non-magnetic environment. Using the Wenner method of measuring a resistance in terms of a mutual inductance and a rate of rotation, their work gave a value of 0.999 994 absolute ohm for the new as-maintained unit of resistance at NBS. The mean value assigned to 10 Thomas-type standard resistors from this experiment was found to have been the same between 1938 and 1948 to within 1 / . As Thomas et al. wrote in the 1949 paper, this was “the first satisfactory method that has been devised for checking the stability of the unit as maintained by a group of wire-wound resistors.” From 1901 to 1990, the U.S. Legal Ohm was maintained at 1 by selected groups of manganin resistance standards. Four different types of resistance standards have been represented in these groups, whose numbers have varied from 5 to 17 resistors. From 1901 to 1909, the group comprised Reichsanstalt-type resistance standards made by the Otto Wolff firm in Berlin. These standards were not hermetically sealed and consequently underwent changes in resistance as a function of atmospheric humidity. In 1907 Rosa at NBS solved the problem by developing a standard whose resistance element is sealed in a can filled with mineral oil [4]. The U.S. representation of the ohm was maintained by 10 Rosa-type 1 resistance standards from 1909 to 1930. Over the years, measurements of differences between the individual Rosa-type resistors indicated that the group mean was probably not constant. In 1930, Thomas reported on the development of his new design for a resistance standard having improved stability [2]. The Thomas resistance standards were more stable immediately following construction than the Rosa-type resistors and two were added to the