B M Wood

A determination of Planck's constant using the NRC watt balance

We have measured Plancks constant and have obtained a value of 6.626?070?34(12)???10?34?J?s. To our knowledge this measurement of h has the lowest uncertainty reported to date. This result has been obtained from measurements of four masses of different material and nominal values varying from 1?kg to 250?g. The experimental procedures and the measurement uncertainties are described in detail.

The quantized Hall resistance: towards a primary standard of impedance

In this paper, it is shown that a quantum Hall resistor (QHR) which exhibits a proportionality relationship between the deviation of the Hall resistance from RK/i and the ac dissipation in the system, represented by ρxx, can be used as a primary standard of ac resistance. As an example, a calculable quadrifilar resistor was calibrated against the QHR at kilohertz frequencies. The agreement between the calibration using the QHR and the calculated frequency dependence of the quadrifilar resistor is better than 4 parts in 10 8 at 1 kHz. This result is achieved despite the frequency and current dependence of the ac-QHR. The most important criterion to achieve accurate measurements using an ac-QHR standard is to extrapolate the value of the Hall resistance to zero dissipation.

A review of Josephson comparison results

In March 2009, the Consultative Committee for Electricity and Magnetism (CCEM) reviewed and re-endorsed its recommendations originally proposed during its 25th meeting (April 2007) for changes in the SI electrical units (Recommendation E-1). Among its considerations, the document stated that the representation of the volt using the Josephson effect and the conventional value of the Josephson constant, KJ−90, has provided practical, accessible, reproducible, low noise and highly linear references worldwide since 1990. We summarize the measurement results of comparisons between Josephson voltage standards that help support this statement, especially concerning the accuracy and the simplicity of the Josephson relationship between voltage and frequency. We also detail a list of influence parameters and the ranges over which the Josephson relationship has been tested.

Direct determination of capacitance standards using a quadrature bridge and a pair of quantized Hall resistors

We describe measurements made using a quadrature bridge to simultaneously compare two quantum Hall resistors (QHRs) with two 10 nF capacitors at 1233 Hz. This direct approach eliminates uncertainties due to thermal and structural instabilities and to AC/DC differences of the wire resistors normally used with an AC ratio bridge as an intermediary stage.

AC longitudinal and contact resistance measurements of quantum Hall devices

A new coaxial measurement system has been developed to investigate the ac longitudinal resistance along the high-potential side of the quantum Hall resistance (QHR). A novel equivalent circuit of the QHR is used to analyse the ac measurements of the longitudinal resistances along both the low- and high-potential sides of the QHR sample. In addition, a bridge for the measurement of ac contact resistances of the sample is presented. For the first time, it is now possible to perform all the ac measurements whose dc equivalents are well established for reliable dc quantum Hall measurements. While the ac longitudinal resistances on the high- and low-potential sides of the sample are very similar, interesting differences have been observed at high resolution.

A useful reflection

We discuss a simple, practical transformation between the probability density function (PDF) of the measurand and the associated PDF of the process for producing measurement results, both of which are used in metrology. This transformation is a reflection through the reported value and captures the essential reversal of skewness of these two perspectives for one-dimensional cases (i.e. any scalar quantity with no free parameters). It is useful for reconciling the skewness of the measurand and measurement-results perspectives, identifying cases where the choice of perspective should not matter and helping to treat the exceptional cases, involving PDF asymmetry, where it can matter.

CCC Bridge with digitally-controlled current sources

We have built a cryogenic current comparator (CCC) resistance bridge for the comparison of standard resistors in the range of 1 Omega-100 kOmega. The use of fully programmable current sources allows for smooth polarity reversals, regardless of the time constant of each side of the bridge. We measured a CCC transfer function of 2.25 muA middotturn/phi0 and a noise level of 54 pA middot turn/radic(Hz) at 1 Hz, with a 1/f corner of about 0.6 Hz. Two different readout schemes were implemented and compared with good agreement. We performed a few measurements to assess the accuracy of the bridge and evaluated all the significant sources of error. The total uncertainty of the bridge is estimated to be 2.6 parts in 109 between 1 Omega and 10 kOmega.

Outlier rejection for the weighted-mean KCRV

We propose new criteria to help select a subset of peer results for determining a weighted-mean Key Comparison Reference Value (KCRV) for describing Key Comparisons supporting the CIPM Mutual Recognition Arrangement (MRA). Often the results are all well behaved: their scatter is in the range expected from the Gaussian probability distributions associated with their stated standard uncertainties. In this case, the KCRV would usually be the inverse-variance weighted mean of all eligible results, the maximum likelihood estimate of independent Gaussian probability distributions. If most—but not all—of the results are well behaved, the comparison may be better described in terms of a KCRV computed with ‘outliers’ excluded from the weighted mean. An outlier’s effects are quantified in terms of the logarithmic slope and logarithmic curvature of its associated probability density function in the vicinity of the consensus value. The outer tails of the probability distributions are generally not known with confidence, so two new criteria are suggested to help justify the identification of outliers: one based on uncertainty in the logarithmic slope, and the other based on uncertainty about whether the logarithmic curvature is negative definite.

Direct comparison of the NRC hysteretic and the NRC watt balance Programmable Josephson Voltage Systems

At the National Research Council in Ottawa, the conventional Hysteretic Josephson Array Voltage System (HJVS) and the Programmable Josephson Array Voltage System (PJVS) associated with the watt balance experiment have been directly compared. This direct comparison of Josephson array systems uses an improved automated biasing technique for the hysteretic array. The comparison is being performed with different nanovoltmeters including the unique detector associated with the watt balance experiment. Noise, stability and linearity of the detectors are examined. Preliminary results indicate that the two Josephson systems, under optimized conditions, agree within (0.09 Ñ 0.54) nV at the nominal level of 1 V.

The NRC watt balance project

In 2009 NRC purchased the NPL watt balance and associated equipment and shipped it to a new purpose built laboratory in Ottawa, where the balance is being reassembled. In this paper we will describe the laboratory, give details of the rebuilding program and outline our plans and a tentative timetable for the determination of a value for Plancks constant (h).