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New Realization of the Ohm and Farad Using the NBS Calculable Capacitor

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New Realization of the Ohm and Farad Using the NBS Calculable Capacitor IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 38, NO. 2, APRIL 1989 249 New Realization of the Ohm and Farad Using the NBS Calculable Capacitor JOHN Q. SHIELDS, RONALD F. DZIUBA, MEMBER, IEEE,AND HOWARD P. LAYER Abstrucl-Results of a new realization of the ohm and farad using part of the NBS effort. This paper describes our measure- the NBS calculable capacitor and associated apparatus are reported. ments and gives our latest results. The results show that both the NBS representation of the ohm and the NBS representation of the farad are changing with time, cNBSat the rate of -0.054 ppmlyear and FNssat the rate of 0.010 ppm/year. The 11. AC MEASUREMENTS realization of the ohm is of particular significance at this time because The measurement sequence used in the 1974 ohm and of its role in assigning an SI value to the quantized Hall resistance. The farad determinations [2] has been retained in the present estimated uncertainty of the ohm realization is 0.022 ppm (lo) while the estimated uncertainty of the farad realization is 0.014 ppm (la). NBS measurements. A 0.5-pF calculable cross-capacitor is used to measure a transportable 10-pF reference capac- itor which is carried to the laboratory containing the NBS I. INTRODUCTION bank of 10-pF fused silica reference capacitors. A 10: 1 HE NBS REPRESENTATION of the ohm is bridge is used in two stages to measure two 1000-pF ca- T based on the mean resistance of five Thomas-type pacitors which are in turn used as two arms of a special wire-wound resistors maintained in a 25°C oil bath at NBS frequency-dependent bridge for measuring two 100-kQ Gaithersburg, MD. Similarly, the NBS representation of resistors. A 100: 1 bridge is used to compare each of the the farad FNss is based on the mean capacitance of four two 100-kQ resistors with a 10004 transportable resistor, fused-silica capacitors in a comparable oil bath. By means R3 11, which is carried to the laboratory containing the of the United States calibration hierarchy, measurements NBS bank of l-Qresistors where the dc stepdown is made. of resistance, capacitance, and inductance made through- The ac-dc difference of R311 is determined by means of ' out the country are generally traceable to these represen- a special 1000-Q coaxial resistor of negligible ac-dc dif- tations. ference. All ac measurements are carried out at w = lo4 Realization of the ohm and farad is necessary for two rad/s 2: 1592 Hz. distinct reasons: first, to determine QNBSand FNssin SI The calculable capacitor [2], the ac bridges [3] and units , thereby ensuring that measurements based on these standards [4], the ac-dc resistance standard [5],and the electrical quantities are consistent with the SI, the unit equipment used to measure transformer ratios [6],[7] and system used throughout the world; second, to determine voltage dependencies [8] remain basically the same as in in SI units a number of fundamental physical constants of the 1974 measurements. The calculable capacitor was importance to both physics and electrical metrology. partially disassembled in order to realign the electrical and These include the fine-structure constant a,the quantized optical axes, clean the optical flats, install larger diameter Hall resistance R, = h/e2, and the Josephson frequency PTFE rings on the guard tubes, check for microphonic to voltage quotient E, = 2e/h (h is the Planck constant, coupling errors [9], and measure the distributed induc- e is the elementary charge). Indeed, it is likely that start- tances and capacitances used to calculate frequency cor- ing January 1, 1990, representations of the ohm world- rections. wide will be based on a conventional value of RH and rep- Residual gas in the calculable capacitor has been re- resentations of the volt will be based on a new conven- duced to a negligible level by the installation of a turbo- tional value of E,, both consistent with the SI. These molecular pump. All of the ac voltage sources and some values are to be derived from the data available by June of the preamplifiers and phase-sensitive detectors are new. 15, 1988 [l]. An automated data acquisition system is now used in the In order to contribute to the pool of data, NBS, like comparison'of the calculable capacitor with the 10-pF ref- other national standards laboratories, is carrying out ex- erence capacitor, resulting in a standard deviation of 0.003 periments to determine RHand E,. Realizing the ohm by ppm for the random scatter in one complete measurement means of the NBS calculable capacitor is an important requiring about one hour. Manuscript received June 10, 1988. J. Q. Shields and R. F. Dziuba are with the Electricity Division, Na- 111. DC MEASUREMENTS tional Bureau of Standards, Gaithersburg, MD 20899. Relative to the 1974 measurements the most significant H. P. Layer is with the Length and Mass Division, National Bureau of Standards, Gaithersburg, MD 20899. reduction in uncertainty is in the dc stepdown. To relate IEEE Log Number 8826209. the 1000-Q transportable resistor R3 1 1 to the reference U.S. Government work not protected by U.S. copyright 250 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 38, NO. 2. APRIL 1989 bank of 14resistors, measurements at 1, 100, and 1000 TABLE I Q are necessary using three different measurement sys- UNCERTAINTIESIN THE MEASUREMENTOF FNss IN SI UNITS tems. NBS-built Hamon-type resistance transfer stan- one standard deviacion dards [lo] are used to provide accurate 1 : 100 and 1 : 10 estimate (ppm) resistance ratios to extend the 1-Q reference bank to the Type A (random) 0.003 100-Q level, and then to the 1000-0 level. All of the re- Geometrical imperfections in the calculable capacitor 0.007 sistance standards used in the scaling process to measure Laser/interferometer misalignment 0.003 resistor R3 11 are measured in situ, immersed in specially Frequency (loading) corrections 0.004 designed circular oil baths with temperature maintained at Microphonic coupling 0.005 (25.000 f 0.002)"C. An automated direct-current comparator system [I I] is Voltage dependence 0.005 used to compare the five 1-0 resistors in the reference bank Drift between calibrations/failure to close 0.006 to Hamon transfer standard HlOA containing ten 104 Transformer ratio measurement 0.002 coils configured in the parallel mode. The resistance coils Bridge linearity and phase adjustment 0.003 for Hamon HlOA are individually sealed in brass cans Detector uncertainties 0.002 filled with silicone heat-sink compound and terminated Coaxial choke effectiveness 0.001 with mercury-wetted contacts. The connectors for the par- Temperature corrections for 10 pF capacitors 0.002 allel configuration consist of (a) low-resistance amalgam- ated copper shorting bars for the current terminations, and RS S 0.014 (b) one of two fixtures having fan resistances of either 1 or 10 Q for the potential terminations. A second Hamon standard of similar design but different construction is The Fabry-Perot (flat plate) interferometer is illuminated used as a check standard for scaling from 1 to 100 Q. by a mode-stabilized He-Ne local oscillator whose wave- The measurements at 100 Q are made using an auto- length is stable to within a few hundred kilohertz over the mated direct-current comparator system similar in design, course of a measurement. Because this laser is not stabi- construction, and operation to the 1-Q system. Hamon lized to a well-defined wavelength and may drift several H 10A in its series mode is compared to a guarded Hamon megahertz per day, its frequency is continuously com- standard HlkA containing ten 1000-Qelements in its par- pared to an iodine stabilized laser whose wavelength is allel configuration. The card-type resistance elements for accurate to about three parts in 10" and is stable to sev- HlkA are sealed in a thick-walled aluminum box filled eral parts in 1OI3 [ 121. The lasers are compared by mea- with silicone fluid. The resistor terminations are British suring the frequency of the heterodyne signal using a high- Post Office (BPO) connectors. A second Hamon standard speed photo detector, electronic amplification, and a dig- containing ten 100-Qresistors is used as a check standard ital frequency counter. The frequency comparison con- for scaling from 100 to 1000 Q. tributes no significant error to the laser calibration. Resistor comparisons at the 1000-Q level are made using The interferometer cavity is isolated from the working a guarded, resistance-ratio bridge. The adjustable part of laser by using an acousto-optic modulator which shifts the the bridge is a modified direct-reading, double-ratio set. frequency of the incident beam by 90 MHz and any light With this bridge the individual resistance sections of Ha- reflected back from the interferometer system by an ad- mon HlkA are compared to the 1000-Q transportable re- ditional 90 MHz. Because of the narrow bandwidth of the sistor R311. Alternatively, Hamon HlkA can be com- laser cavity, the return light, shifted from the incident light pared to resistor R311 by first measuring the series- by 180 MHz, is no longer resonant in the laser cavity and parallel combination of its first nine coils, and then mea- is not coupled to the laser modes. No frequency pulling suring its tenth coil individually. of the working laser wavelength due to back reflections A set of three measured values is obtained for resistor from the interferometer or the beam steering optics could R311 over a period of three consecutive days.
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