A summary of the commonalities and highlight of the main differences between watt balance experiments, as well as the history and progress on accurateof thewere published in a recent paper.In Secs. II–IV , the recently built NIST watt balance apparatus is described, as well as the procedures to align the instrument, theand the data analysis. Thevaluefrom December 2015 to January 2016 is reported in Secs. V and VI

In addition to the efforts at NISTand NPL,other national metrology institutes have taken the challenge to build watt balance experiments. These include the Swiss Federal Institute of Metrology (METAS),the Laboratoire National de Métrologie et d’Essais (LNE) of France,the Bureau International des Poids et Mesures (BIPM),theStandards Laboratory of New Zealand (MSL),and the Korea Research Institute of Standards and Science (KRISS).The National Research Council (NRC) of Canada purchased the NPL apparatus in 2011 and made improvements that lowered the overall uncertainty.The National Institute of Metrology (NIM) of China proposed a Joule balance, in which the derivative of the flux with respect tois calibrated using a mutual inductance method instead of moving thein theThe principle of the watt balance experiment is presented in the simplified scheme of Fig.. Some common main elements in most watt balance experiments are the following:

A precision resistor with low temperature and power coefficients. The resistance value is chosen based on the design choices of test mass, field strength, coil length, and setting range of the programmable voltage standard. Common values are 100 Ω or 200 Ω. The absolute value of the resistance is calibrated by comparison to a quantum Hall based resistance.

Precision digital voltmeters as null detectors to measure the deviation of the voltage drop across the resistor in the force mode and the induced voltage of the moving coil in the velocity mode from the voltage set by the Josephson array.

A Josephson array as the voltage standard. It can be a conventional hysteretic array or a programmable array. Most watt balances use a programmable Josephson voltage standard (PJVS), which offers the possibility to track the voltage of the coil in the velocity mode of the experiment during acceleration and deceleration to avoid overloading of the null detector. A PJVS requires a bias current source that should be electrically isolated to operate properly with the watt balance.

Precise laser interferometer to measure the vertical velocity of the coil in motion derived from absolute measurement of length and time interval. Typically, heterodyne and homodyne laser interferometers in a Michelson configuration are used, but a Fabry-Pérot interferometer can also be used as in METAS watt balance.

to determine the local due to gravity. The transfer to the location of the test massis usually done with a relative gravimeter. To date, two different kinds of precise absolute gravimeters are employed in watt balance experiments: a classicaltheof a free falling retroreflectoror an atom

sensitive to forces along the direction of the gravitationalIt can be a pulley, a beam balance, or a commercial mass comparator. Two kinds of mechanical pivots are used: knife edge or flexure. A knife edge can provide a larger angle of rotation than a flexure with the same lever arm length. Hence, if a knife edge is used, the force comparator can play both roles that of the weighing and moving mechanism. Using one mechanism for moving and weighing not only simplifies the design but also has a significant technical advantage. The errorinmode correspond to virtual errorin force mode. This symmetry significantly reduces the effect of these erroron the final result, see Ref.for details. The disadvantage of using a knife edge is that the large mechanical stress introduced in themode can cause time and position dependent mechanical forces in the force mode, known as mechanical hysteresis. The hysteresis is reduced by an erasing procedure where the pivot is exercised in a damped sinusoidalafter each excursion. Flexures have small hysteresis if theare limited, but a separate mechanism to move thein theis required in themode, which makes the mechanical design more complex.

, where thesystem is generally built using rare earth magnets with iron yokes but also can be an electromagnet with room temperature copper wires or superconducting wires as well as a hybrid. As recognized by Olsen,it is advantageous that (a) thedensityhas a spatial gradient inversely proportional to theradius. This eliminates systematic effects associated with geometry changes of thecaused by resistive heating in the force mode, and (b) the radialdensity is uniform along the verticali.e., the force on theis independent of its vertical position. Both goals can be achieved by using two vertically separated electromagnet sources with a symmetry about the mid-plane. Most contemporary watt balance experiments have adopted this geometry, replacing the fieldas proposed originally with permanent magnets as suggested by the watt balance group at BIPM.system comprising permanent magnets with yokes presents two advantages: first, theis contained, and second, theplaced in the air gap of thesystem is shielded from the outside. In most recent papers, the derivative of the flux with respect tois represented in its integral form. It is equal to the product of thedensityand the wire lengthof the). The productwill be referred in this paper as the flux integral.

Combining Eqs. (4) (6) yields the expressionthat can be simplified intoThis is the main watt balance equation. Thevaluein SI units is determined from the ratio of the virtual mechanical powerin SI units to the electrical power in conventional units. In order to reach relative uncertainties on the order of a few parts in 10, the principle quantities in the experiment must be determined with relative uncertainties of a few parts in 10. This is achieved by using, directly or indirectly, five standards: mass, voltage, resistance, time, and length.

However, there is a small subtlety that has to be considered. Bothanddepend on the elementary charge and theOur best estimate of the numerical values of these constants is subject to change depending on new research. In order to avoid changing the calibration of electrical equipment every time a new set of fundamental constants is recommended, the electrical metrology community decided to use a set of constants fixed at the 1990 values.These constants, denoted with the subscript 90, are≡ 483 597.9 × 10Hz Vand≡ 25 812.807 Ω. Both of these constants are used to realize the so called conventional units. The electrical power can now be written aswhere the quantity in {}denotes the numerical value in conventional units.

Replacingandin Eq. (4) with the expressions from the Josephson effect (twice) and the quantum Hall effect yieldsConveniently, the elementary charge cancels out in the product. The quantity in {}denotes the numerical value of power expressed in its SI unit, the watt, W.

The quantum Hall effect occurs in a sample that confines electronic current to two dimensions at sufficiently low temperatures. If such a sample is exposed to a high magnetic field, the quotient of the transverse voltage to the current flowing in the device, is quantized. This quantum Hall resistance is equal to an integer fraction of R K = h / e 2 . The constant R K is named after von Klitzing and the quantum Hall resistance standards are realized in terms of the von Klitzing constant, R = R K / p , where p is an integer.

In the Josephson effect, a tunnel junction between two superconducting materials is radiated with microwave radiation with frequency f U . The potential difference between the two superconductors is given by K J − 1 f U . Here, K J denotes the Josephson constant given by K J = 2 e / h , where e is the elementary charge and h the Planck constant. By using n junctions in series, a larger voltage U = n K J − 1 f U can be obtained.

After the discovery of the quantum Hall effect in 1980 by von Klitzing,and two decades after the prediction of superconductive current tunneling between two metals in 1962 by Josephson,national metrology institutes started realizing the ohm and the volt with very high precision using these effects.

The derivative of the flux with respect tois eliminated when combining both equations together leading towhich describes an equivalence between virtual mechanical and electrical power. Since power isin its SI unit, the watt, the experiment and the apparatus came to be called a “watt balance.”

The watt balance is both an instrument and an experiment, and has its origins in the earliest efforts to create a fundamental electrical standard for the ampere, the base unit of electrical current in the international system of units (SI).It helps to recall the SI definition of the ampere: “The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of a negligible circular cross section, and placed 1 meter apart in a vacuum, would produce between these conductors a force equal to 2 × 10N/m of length.” This base electrical unit is defined in terms of a mechanical quantity, force, which must beusing standards of length, time, and mass, just as originally suggested by Maxwell.Ampere balance experiments, and eventually watt balance experiments, were conceived as a means of carrying out suchusing a mechanical balance to compare an electromagnetic force produced by ain ato the weight of a test mass, i.e.,whereis the mass of the test mass,the local gravitationalthe current in theand Φ thethrough theThe point of the experiment is to derive the value ofin terms of the other quantities, which are mechanical in nature. In the original ampere balances, the flux was produced by a secondThe calculation of the flux from thiswhich is the integral of thedensityover the area segment d, required absoluteof thephysical dimensions, which was difficult and limited the precision that could be achieved. In 1976, Kibblepublished a fundamental insight that significantly changed the field: The vertical derivative of thecould be calculated as the quotient of the induced voltageto the verticalwhen theis moved vertically in the sameThe essence of this insight is that a motor can be used as a generator, and the coupling constant between force and current is the same as the one between voltage andQuickly after Kibble’s proposal, the National Bureau of Standards (NBS, presently NIST) and the National Physical Laboratory (NPL) in the United Kingdom built balances to accommodate a moving mode.

II. NIST’S FOURTH GENERATION WATT BALANCE APPARATUS Section: Choose Top of page ABSTRACT I. BACKGROUND ON WATT BAL... II. NIST’S FOURTH GENERAT... << III. ALIGNMENT PROCEDURES IV. MEASUREMENT PROCEDURE... V. EXPERIMENTAL RESULTS VI. PLANCK CONSTANT DETER... VII. EPILOGUE-2016 REFERENCES CITING ARTICLES

2 3 magnet in four different configurations: (1) with the vacuum chamber top lid open, (2) closed, (3) during the evacuation of the chamber and (4) in vacuum using an autocollimator with the beam reflecting from a mirror on the tear drop plate. Between these four cases, the angle between the top of the magnet and a horizontal plane did not change by more 30 μrad. The fourth generation watt balance apparatus, abbreviated NIST-4, shown in Figs.and, is located inside a radio-frequency shielded laboratory, 12 m underground. For later reference, observe that the balance is oriented along a diagonal with respect to the compass direction of North as shown in the figure, with the main pivot axis falling along a line oriented from the north-west to south-east corners of the room. The room itself is temperature stabilized using an air jacket between the inner and outer exterior walls. Inside the room, the apparatus sits on the center of a concrete block that is 4 m long and 4 m wide and has a total mass of 67 metric tons. This block is isolated from the building’s foundation and, for additional vibration isolation, can be floated off the ground using eight air springs. The apparatus is housed inside a 1.6 m diameter and 2 m tall stainless steel vacuum chamber which is pumped to a vacuum pressure on the order of 0.1 mPa. The watt balance apparatus is supported at three points in a pseudo-kinematic fashion on top of two stainless steel, sand-filled, parallel tubes that run from south-west to north-east through the chamber. The tubes are structurally decoupled from the vacuum chamber via four flexible bellows. This minimizes the transmission of chamber vibrations to the apparatus and maintains the apparatus alignments from air to vacuum. We have verified the alignment of the top of thein four different configurations: (1) with the vacuum chamber top lid open, (2) closed, (3) during the evacuation of the chamber and (4) in vacuum using an autocollimator with the beam reflecting from a mirror on the tear drop plate. Between these four cases, the angle between the top of theand a horizontal plane did not change by more 30 μrad.

35 106, 627 (2001). 35. J. P. Schwarz, R. Liu, D. B. Newell, R. L. Steiner, D. B. Newell, and E. R. Williams, J. Res. Natl. Inst. Stand. Technol., 627 (2001). https://doi.org/10.6028/jres.106.028 36 51, S114 (2014). 36. I. M. Choi and I. A. Robinson, Metrologia, S114 (2014). https://doi.org/10.1088/0026-1394/51/2/S114 33 2014 Conference on Precision Electromagnetic Measurements ( IEEE , 2014), p. 364. 33. L. S. Chao, F. Seifert, A. Cao, D. Haddad, D. B. Newell, S. Schlamminger, and J. R. Pratt,, 2014), p. 364. https://doi.org/10.1109/CPEM.2014.6898410 motion of the flat, that can be coarse positioned with adjustment screws in air. Two stepper motors provide fine positioning of the flat with a total range of about 0.5 mm in the x and y direction. The coil moves with the flat. With these two motors, the operator can adjust the position of the coil in the magnetic circuit while the apparatus is in vacuum. The main balance element is an aluminum wheel with a total mass of 18 kg and a diameter of 610 mm. The wheel was plated with electroless nickel and subsequently diamond turned. This resulted in a maximum deviation of 900 nm from roundness. The wheel pivots about a tungsten-carbide knife edge. A gimbal structure is suspended off each side of the wheel via multi-filament bands. Each band contains 60 filaments, each 130 μm wide and 317.5 mm long, photo etched from a 381 mm long sheet of 80 μm thick grade four titanium. Below the knife edge is a flat made from polished tungsten carbide and coated with diamond like carbon (DLC). The combination of knife edge and flat materials were selected based on previous NIST research resultsand a recent NPL paper.According to the latter findings, hysteresis performance of the knife edge pivot improves when the knife edge material or coating is softer than the flat. Consequently, the DLC coating was omitted from the knife, in contrast to previous approaches at NIST. The flat is mounted on an extremely large flexure (ELF),guiding the horizontalof the flat, that can be coarse positioned with adjustment screws in air. Two stepper motors provide fine positioning of the flat with a total range of about 0.5 mm in theanddirection. Themoves with the flat. With these two motors, the operator can adjust the position of thein the magnetic circuit while the apparatus is in vacuum.

magnetic field is generated by an 800 kg permanent magnet system measuring 60 cm in diameter and 45 cm in height. Two Sm 2 Co 17 disks oriented in repulsion generate, in a 30 mm wide air gap, a 0.55 T radial magnetic field guided by the iron yoke. 29,30 62, 1524 (2013). 29. S. Schlamminger, IEEE Trans. Instrum. Meas., 1524 (2013). https://doi.org/10.1109/TIM.2012.2230771 63, 3027 (2014). 30. F. Seifert, A. Panna, S. Li, B. Han, L. Chao, A. Cao, D. Haddad, H. Choi, L. Haley, and S. Schlamminger, IEEE Trans. Instrum. Meas., 3027 (2014). https://doi.org/10.1109/TIM.2014.2323138 magnetic flux density is mostly oriented along the radial direction, there is a small vertical component. This vertical field is caused by the finiteness of the gap (fringe fields at the end of the gap) and an asymmetry in the air-yoke boundary. An evaluation of the vertical component for the NIST-4 magnet can be found in Ref. 31 52, 445 (2015). 31. S. Li, J. Yuan, W. Zhao, and S. Huang, Metrologia, 445 (2015). https://doi.org/10.1088/0026-1394/52/4/445 z = ± 50 mm, which would be even smaller (0.05%) in the NIST-4 measurement interval |z| < 25 mm. In the fine alignment of the apparatus, both the radial and vertical magnetic field will contribute. A detailed analysis of the influence of B z in the alignment is described in Ref. 32 53, 817 (2016). 32. S. Li, S. Schlamminger, D. Haddad, F. Seifert, L. Chao, and J. R. Pratt, Metrologia, 817 (2016). https://doi.org/10.1088/0026-1394/53/2/817 magnet system, so that the massive magnet system serves as a rigid structural foundation. Theis generated by an 800 kg permanentsystem60 cm in diameter and 45 cm in height. Two SmCodisks oriented in repulsion generate, in a 30 mm wide air gap, a 0.55 T radialguided by the iron yoke.While thedensity is mostly oriented along the radial direction, there is a small vertical component. This vertical field is caused by the finiteness of the gap (fringe fields at the end of the gap) and an asymmetry in the air-yoke boundary. An evaluation of the vertical component for the NIST-4can be found in Ref.. The ratio of the vertical component and the radial component is at most about 0.6% at= ± 50 mm, which would be even smaller (0.05%) in the NIST-4interval || < 25 mm. In the fine alignment of the apparatus, both the radial and verticalwill contribute. A detailed analysis of the influence ofin the alignment is described in Ref.. The mechanics of the watt balance is mounted directly to the top of thesystem, so that the massivesystem serves as a rigid structural foundation.

Suspended from the north-east side of the balance wheel is the main-mass assembly. A three-pointed aluminum spider connects to the end of the multi-filament band with an intermediate torsion flexure, a 60-strand bundle of 75 μm diameter platinum-tungsten wires. The mass pan has two levels, the main-mass pan and the auxiliary-mass pan. The two levels are rigidly connected by aluminum rods. The mass pan accepts the main mass from a plunger that reaches through a cut-out in the center of the main-mass pan. During velocity mode, the auxiliary-mass pan carries a 0.5 kg mass, named the auxiliary mass. Transitioning the instrument from velocity to force mode requires removal of the auxiliary mass from the pan system. This is accomplished with a lift mounted on a rotary stage below the auxiliary mass pan. Removing the auxiliary mass makes the main-mass side lighter by 0.5 kg. Hence, in order to maintain the wheel’s position, an electromagnetic force of 5 N must be generated by the main coil. Since the mass on the counter-mass side has not changed, the static load supported by the knife edge due to the transition from gravitational to electromagnetic force remains constant. This is a significant change from the previous NIST balance, implemented to improve the performance of the knife edge pivot.

Three carbon fiber rods attach the main coil to the spider leg via monolithic flexures at both ends having two orthogonal degrees of freedom each. A total of six flexures provide four different motions. The coil can tilt around the x or y-axis. For these two motions, the spider tilts with the coil because the three connecting rods are forcing the spider and the coil to be parallel all the time. The other two motions are a shear motion, i.e., the coil translates along the x or y-axis relative to the spider. The first two types of motions occur when a torque is applied to the coil. Horizontal forces on the coil cause shear motions. In reality, all four motions are present. The compliance of the system translates parasitic forces and torques of the energized coil into relatively large horizontal and angular displacements. From these recorded displacements, the parasitic forces and torques can be estimated. Both the mass pan and coil pivot each about independent monolithic flexures with two orthogonal degrees of freedom. Both flexures are nested to have all four axes of rotation coplanar and intersecting in a single virtual pivot. This monolithic flexure minimizes the coupling between the two suspended systems and aligns the electromagnetic and gravitational forces into a single point on the vertical axis.

coil, 945 turns were wound in the center of a monolithic coil former made of 99.5% alumina (Fig. 4 coil has a mean radius of 0.217 m, a resistance of 112 Ω and a low frequency inductance of 6 H. Two additional gradiometer coils were wound above and below the main coil. The gradiometer coils are only used during the assembly of the magnet, where they provide a means to measure the vertical dependence of the radial magnetic flux density in the gap of the magnetic circuit. A shimming procedure has been developed that manipulates the reluctance of the yoke, and the variation of the radial flux density in the precision gap can be systematically adjusted to within less than 1 mT i.e., ΔB r /B r < 2 × 10−4. See Ref. 30 63, 3027 (2014). 30. F. Seifert, A. Panna, S. Li, B. Han, L. Chao, A. Cao, D. Haddad, H. Choi, L. Haley, and S. Schlamminger, IEEE Trans. Instrum. Meas., 3027 (2014). https://doi.org/10.1109/TIM.2014.2323138 coil, the coil former was painted with a static dissipative coating. To prevent closed-circuit eddy current loops, the coating was interrupted on three spots, 120° apart. Each of the three coated sections are grounded individually to eliminate static charge buildup. For the main945 turns were wound in the center of a monolithicformer made of 99.5% alumina (Fig.). Thehas a mean radius of 0.217 m, a resistance of 112 Ω and a low frequency inductance of 6 H. Two additional gradiometerwere wound above and below the mainThe gradiometerare only used during the assembly of thewhere they provide a means tothe vertical dependence of the radialdensity in the gap of the magnetic circuit. A shimming procedure has been developed that manipulates the reluctance of the yoke, and the variation of the radial flux density in the precision gap can be systematically adjusted to within less than 1 mT i.e., Δ< 2 × 10. See Ref.for details on the shimming procedure. Prior to winding thetheformer was painted with a static dissipative coating. To prevent closed-circuit eddy current loops, the coating was interrupted on three spots, 120° apart. Each of the three coated sections are grounded individually to eliminate static charge buildup.

motions of the main coil are monitored using optical techniques. A set of three hollow retroreflectors is installed around the coil circumference 120° apart and form the measurement arms for the fiber-coupled heterodyne interferometry system as depicted in Figs. 5 6 measuring the absolute translation of the optical center of the coil along an axis aligned to gravity. Another set of three optical elements is similarly installed around the coil circumference at 120° intervals, but offset from the first by 60°. This set consists of two solid retroreflectors and a flat mirror. The two solid retroreflectors are used in an optical beam translation scheme to monitor horizontal displacements and angular motion of the coil about the z-axis. The single flat mirror is used with an optical lever scheme to measure angular motions about the x and y axes. The reference light beams in both the beam translation and optical lever setups are amplitude modulated to facilitate lock in detection of the reflected light on position sensitive photo-detectors located on the top of the magnet. The six rigid-bodyof the mainare monitored using optical techniques. A set of three hollow retroreflectors is installed around thecircumference 120° apart and form thearms for the fiber-coupled heterodyne interferometry system as depicted in Figs.and. This system is dedicated tothe absolute translation of the optical center of thealong an axis aligned to gravity. Another set of three optical elements is similarly installed around thecircumference at 120° intervals, but offset from the first by 60°. This set consists of two solid retroreflectors and a flat mirror. The two solid retroreflectors are used in an optical beam translation scheme to monitor horizontal displacements and angularof theabout the-axis. The single flat mirror is used with an optical lever scheme toangularabout theandaxes. The reference light beams in both the beam translation and optical lever setups are amplitude modulated to facilitate lock in detection of the reflected light on position sensitive photo-detectors located on the top of the

Motion of the coil is weakly constrained along all but the z-axis, so servo control loops are required around the other five degrees of freedom during various operations to damp unwanted oscillations at various points during a measurement. The position signals from the previous paragraph are the control inputs. To control the rotations about the main-translation axis z, three glass plates coated with gold are attached vertically to each carbon fiber rod. These plates are sandwiched between sets of high-voltage electrodes fixed to the magnet, as shown in Figs. 2 3 coils are mounted on the coil itself, between the optical elements, 120° apart to actively damp the horizontal and angular motions about x and y. Each mini coil set contains one vertically and one horizontally oriented coil. The damping coils are wound on carbon-filled, static dissipative polyether ether ketone (PEEK) coil formers, with a diameter of 18 mm. They are driven individually using single-ended, bipolar, 16-bit digital-to-analog converters. However, two coils in one set share a ground to minimize the wire counts. Hence, only three wires for each set are required. The output channels are calculated by software to excite pure horizontal or angular motion about x and y axes. of theis weakly constrained along all but the-axis, so servo control loops are required around the other five degrees of freedom during various operations to damp unwanted oscillations at various points during aThe position signals from the previous paragraph are the control inputs. To control the rotations about the main-translation axis, three glass plates coated with gold are attached vertically to each carbon fiber rod. These plates are sandwiched between sets of high-voltage electrodes fixed to theas shown in Figs.and, to form a bi-directional electrostatic actuator. To actuate the remaining degrees of freedom, three sets of miniare mounted on theitself, between the optical elements, 120° apart to actively damp the horizontal and angularaboutand. Each miniset contains one vertically and one horizontally orientedThe dampingare wound on carbon-filled, static dissipative polyether ether ketone (PEEK)formers, with a diameter of 18 mm. They are driven individually using single-ended, bipolar, 16-bit digital-to-analog converters. However, twoin one set share a ground to minimize the wire counts. Hence, only three wires for each set are required. The output channels are calculated by software to excite pure horizontal or angularaboutandaxes.

A variety of electrical connections to the instrument are required, the most difficult being those that must reach the main coil and damping coils, since these wiring connections can impart parasitic rotations of the coil about x and y axes and rotation of the balance wheel. Eleven wires, nine for the damping coils and two for the main coil, are routed inside the carbon fiber tubes that connect the coil to the spider. The distance from the spider to the wheel, about 30 cm, is bridged using finer wires. The wires terminate on blocks mounted on the balance wheel such that the wires are coplanar with the bands running off the wheel. The electrical signals are carried with thicker wire along the radius vector of the wheel to its center. Fine wires are used again to bridge from the moving wheel to the fixed wheel support. Here the eleven wires are arranged such that they are along a line extending from the knife edge ridge. The wire routing was chosen to minimize torques produced by the wires about the knife-edge and to keep this torque independent of wheel angle.

The motors that operate the mass lift dissipate heat during the weighing cycle. A thick copper bar is installed on top of the magnet as a thermal link to transport unwanted heat to the outside.

Suspended from the south-west side of the wheel is the counter-mass assembly. This structure serves both as a tare weight and as the velocity mode driving motor. The motor consists of a small coil wound on a G10 fiberglass former and a radial magnetic field generated by two neodymium disks bound by iron yoke, a smaller but similar design to the main magnet. A large aluminum cylinder forms the top part of the counter-mass assembly. A corner cube is mounted facing down on the lower side of the cylinder. This corner cube terminates the measurement arm of an interferometer whose readout is used as an input for the balance feedback.

For all four heterodyne interferometers, three on the main side and one on the counter-mass side, time interval analyzers measure the zero-crossings of the reference and the measurement signals. Along with the time interval measurements, continuous event counts of the reference and the measurement signals is maintained on all channels in order to keep track of the coil position during the measurement.

34 2012 Conference on Precision Electromagnetic Measurements ( IEEE , 2012), p. 336. 34. D. Haddad, B. Waltrip, and R. L. Steiner,, 2012), p. 336. https://doi.org/10.1109/CPEM.2012.6250939 I, flowing through the coil to maintain the position of the balance in the force mode. A similar current source is used for the counter-mass motor in the velocity mode. Both current sources are floating, since they are battery powered and connect to the control computer via and optical fiber link. They feature low noise on the order of 100 pA / Hz at 1 Hz and a relative short-term drift less than 0.1 (nA/mA)/h. A custom built programmable current sourceoperates in a feedback loop to generate the necessary current,, flowing through theto maintain the position of the balance in the force mode. A similar current source is used for the counter-mass motor in themode. Both current sources are floating, since they are battery powered and connect to the control computer via and optical fiber link. They feature low noise on the order of 100at 1 Hz and a relative short-term drift less than 0.1 (nA/mA)/h.

velocity mode shown in Fig. 7 measurement. To reconfigure the electrical circuit between force andmode shown in Fig., a switchbox, featuring low thermal electromotive force (EMF) latching relays are used. The relays are only energized during mode switching and the circuit is completely turned off during the

40,41 54, 616 (2005). 40. Y. Chong, C. J. Burroughs, P. D. Dresselhaus, N. Hadacek, H. Yamamori, and S. P. Benz, IEEE Trans. Instrum. Meas., 616 (2005). https://doi.org/10.1109/TIM.2004.843080 49, 635 (2012). 41. Y. Tang, V. N. Ojha, S. Schlamminger, A. Rüfenacht, C. J. Burroughs, P. D. Dresselhaus, and S. P. Benz, Metrologia, 635 (2012). https://doi.org/10.1088/0026-1394/49/6/635 42 71, 1866– 1868 (1997). 42. S. P. Benz, C. A. Hamilton, C. J. Burroughs, T. E. Harvey, and L. A. Christian, Appl. Phys. Lett., 1866–(1997). https://doi.org/10.1063/1.120189 A 2.5 V primary voltage system based on a NIST-fabricated programmable array of Josephson junctions and developed electronicsis used as the programmable Josephson voltage standard (PJVS).The leakage resistance to ground of the non-battery powered bias electronics was improved with an air gap isolation transformer.