Possible Changes in the U.S. Legal Units Of Voltage and Resistance

The Consultative Committee on Electricity of the International Committee on Weights and Measures is considering adopting sometime in the future 1) a new value for the Josephson frequency-voltage ratio 2e/h (e is the elementary charge and h is the Planck constant) and 2) a value for the quantized Hall resistance R _H≡h/e ². Both values are to be chosen as consistent with their International System of Units (SI) values as possible and would be used by every national standards laboratory which employs the Josephson and quantum Hall effects to define and maintain their national or legal units of voltage and resistance. Based on current knowledge, this would lead to an increase in the U.S. Legal Volt of about nine parts-per-million (ppm) and an increase in the U.S. Legal Ohm of about 1.5 ppm. Comparable changes would be required in the voltage and resistance units of most other national, governmental, and industrial standards laboratories throughout the world. Many high-precision instruments would also have to be readjusted to make them consistent with the new units. It is the purpose of this paper to review in some detail the basis for these proposed and potentially significant changes.