A REALIZATION OF THE SI WATT BY THE NPL MOVING-COIL BALANCE

Mechanical and electrical power in SI units have been equated by measurements made on a coil part of which is in a strong magnetic field. The force due to a current I flowing in the coil, is weighed by opposing it with a mass M subject to the earth's gravitational acceleration g. This is combined with a separate measurement in which a voltage V is generated in the coil when it is moved vertically with velocity u through the relationship IV = M g u. If the current produces a voltage V across a resistor whose value R is known in SI units [1], then V = (M g u R)1/2. Hence the voltage V and the current I are known in SI units and can be used to express the value of the NPL working standards in SI units. The working standard of voltage has hitherto been maintained in terms of a Josephson effect apparatus by ascribing the value 483 594 GHz/volt (maintained) to the Josephson constant K(J) presumed equal to 2e/h [2]. The measurements reported here suggest a different value of K(J) 483 597,903 +/- 0,035 ought to be used, based on the premise that the SI value of the quantum Hall resistance is R(K) = 25 812,8092 +/- 0,0014 OMEGA [3]. If one presumed also that R(K) = h/e2 exactly, the values of elementary charge e and the Planck constant, h, [4] which may be deduced from these measurements are e = 1,602 176 35 +/- 0,000 000 14 x 10(-19) C, h = 6,626 068 21 +/- 0,000 000 90 x 10(-34) J s, which may be compared with the values recommended by the CODATA Task Group on Fundamental Constants [5] which are e = 1,602 177 33 +/- 0,000 000 14 x 10(-19) C, h = 6,626 075 5 +/- 0,000 004 0 x 10(-34) J s.