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Power Loading Effects in Precision 1 Ohm Resistors



George R. Jones, Randolph E. Elmquist


Five manganin alloy Thomas-type 1 Ohm resistors serve as primary working standards at the National Institute of Standards and Technology (NIST) in the precision potentiometer direct current comparator (DCC) system used for special 1 Ohm customer calibrations. To maintain and predict the values of these resistors, the value of this bank is compared to the quantized Hall resistance (QHR) standard at NIST approximately twice a year. This is accomplished through the use of several precision 1 Ohm resistors manufactured from 1975 through 1992 by the Australian National Measurement Laboratory (NML), using the resistance alloy Evanohm. Over many years of careful monitoring, the relative values of these transfer resistors were seen to have discrepancies that were not related to the drift in the value of the primary working standards and exceeded the type A, statistically derived uncertainty in the measurement systems. Some of these variations were believed to be due to power loading in the transfer resistors. Recent experiments on different types of precision 1 Ω resistors have demonstrated that conditions of power dissipation within the resistors and the duty cycle of the power applied to the resistors can have significant effects on the uncertainty of the measurements. This paper describes the experimental results and measurement uncertainty due to these power loading effects. Additional measurements have examined loading effects in Thomas-type and certain other precision 1 Ω resistors. The relationship between the loading effect and the temperature coefficient will be described, as well as possible temperature gradient contributions to the changes of resistance observed in these measurements.
Conference Dates
August 3-7, 2008
Conference Location
Orlando, FL
Conference Title
NCSL International Workshop and Symposium


precision, resistor, power, loading


Jones, G. and Elmquist, R. (2008), Power Loading Effects in Precision 1 Ohm Resistors, NCSL International Workshop and Symposium, Orlando, FL, [online], (Accessed March 5, 2024)
Created August 1, 2008, Updated February 17, 2017