– the intergovernmental organization through which Member States act together on matters related to measurement science and measurement standards. 



The BIPM Kibble (watt) balance: General conceptThe international metrology community renamed their movingcoil watt balances as 'Kibble balances' in honour of the pioneering work in this field by Dr Bryan Kibble, following his death in 2016. Most Kibble balances operate in two distinct measurement phases – weighing and moving – so measurement accuracy could be limited by variations in the environmental and experimental conditions between the two phases. The main novelty of the BIPM Kibble balance is its implementation of simultaneous weighing and moving measurement phases, in addition to the conventional approach. The great advantage of this simultaneous measurement is to guarantee that the geometric properties of the coil, the coil orientation and the magnetic field, acting in both phases are strictly the same. The simultaneous method hence makes the experiment less sensitive to changes in magnetic field and alignment when compared to a twophase Kibble balance. In addition, a comparison of measurements carried out using the conventional and the BIPM simultaneous approach may provide a better understanding of the limiting factors and possibly reveal some unexpected effects unique to each technique.
The principle of the Kibble (watt) balanceThe concept of the moving coil watt balance was proposed by B. P. Kibble (NPL) in 1975. The experiment consists of two parts: the weighing and the moving experiments.
If the properties of the coil and the magnetic field, L and B, and their alignment, do not change between the two experiments, they can be eliminated from both equations, which leads to the new equation which shows an electrical power on the lefthand side and a mechanical power on the righthand side. This explains the name of the experiment, since the watt is the unit of power. It is important to understand that both types of power are only "virtual", in the sense that they do not appear in one of the single phases of the experiment. In the case of the electrical power, the voltage is measured in the moving experiment, and the current in the weighing experiment. To establish a link between the macroscopic mass m and the Planck constant h, the electrical quantities voltage and current are measured by using two macroscopic quantum phenomena, the Josephson effect and the quantum Hall effect. The Josephson effect allows us to determine an unknown voltage U as a dimensionless multiple u' of a combination of the Planck constant, the elementary charge and a precisely measurable frequency f_{J}:
The quantum Hall effect allows us to determine an unknown resistance R as a dimensionless multiple r' of a different combination of the Planck constant and the elementary charge:
The application of both effects on the measurement of the voltage and the current in the Kibble balance equation leads to:
where the first component on the righthand side shows the quantities related to the measurement of the voltage and the current using the macroscopic quantum effects, the second component shows the mechanical measurands gravitational acceleration g and velocity v, and the last term is the Planck constant. This equation establishes the link between the macroscopic mass m and the Planck constant h. Suspension
The moving coil hangs from the weighing cell via a variablelength suspension. An integrated electrostatic motor varies the length of the suspension and allows the coil to move (see image below). Force measurements are carried out using a The electrostatic motor is composed of three parallel plate electrodes (see image below). The central electrode is grounded and fixed. The upper and lower electrodes are mobile. By independently modulating the voltages on the two mobile electrodes, a net electrostatic force is obtained and the electrodes move. The movement of the mobile electrodes is transmitted to the lower part of the suspension via three levers separated by 120°.
Electrical systemThe current source
Voltage reference
Optical systemOptical systems based on position sensitive detectors (PSDs) are used to measure the coil displacement for the unwanted degrees of freedom (2 translations and rotations around three axes). The vertical coil velocity is measured with a heterodyne interferometer. This measurement has recently been improved by replacing the oneaxis with a threeaxis interferometer. Moving from a 1axis to a 3axis interferometer has improved the voltage to velocity signaltonoise ratio by a factor of 10.
MagnetA closed magnet circuit is employed. The circuit design includes two discs of Sm_{2}Co_{17} magnets, magnetized in opposite directions, as the flux source. The yoke is made of a high permeability FeNi alloy which eliminates the nonuniformity of the magnetic flux density in the air gap, which would otherwise result from the small magnetization asymmetry of the two permanent magnet discs. The mean diameter of the circular air gap is 250 mm and the width is 13 mm. The magnetic field has radial symmetry and a magnetic flux density of about 0.5 T. Due to the geometry of the system, the magnets and the air gap are completely screened by the high permeability iron yoke. This considerably reduces the level of external electromagnetic perturbations detected by the coil. The symmetry of the circuit with respect to the horizontal plane helps to improve the uniformity of the flux density in the air gap. The uniformity of the vertical profile of the flux density in the air gap was measured to be below 1 × 10^{4} within the working length of 40 mm. A theoretical and experimental study was conducted to investigate the influence of the coilcurrent on the magnetic field. The magnetic flux generated by the coil goes through the magnet and produces an additional contribution to the magnetic field at the coil position. It has been found that this mechanism leads to a significant slope of the vertical magnetic field profile. It has also been demonstrated that the effect on the magnetic field in the velocity measurement is twice that deduced from the weighing measurement. The exact size of the effect depends on the specific geometry and dimensions of each magnetic circuit, but in general has to be taken into account at the present level of uncertainty of Kibble balances. For further details see: Magnetic field alignmentAn experimental procedure has been developed to accurately align the magnetic field of the circuit perpendicular to the direction defined by the local acceleration of gravity. The technique is based on a rotating Hall probe and a highsensitive tiltmeter. The magnetic plane at the central position inside the gap was aligned to better than 10 μrad with an uncertainty below For further details see:
Vibration isolation
The Kibble balance laboratory has two concrete foundations – one for the Kibble balance (64 tonnes) and one for the gravimeter (10 tonnes). Each is isolated from the main floor of the building but remains in lateral contact with the earth to avoid lowfrequency swinging motions (excited by surface Rayleigh waves). The horizontal and vertical rms velocities due to ground vibrations are respectively
In Kibble balances, the absolute value of the local gravitational acceleration g at the mass weighing position should be determined at the 10^{−9} level, corresponding to a few µGal (1 µGal= 1 × 10^{−8} m/s^{2}). The absolute value of g was measured in the BIPM Kibble balance laboratory during the 2009 International Comparison of Absolute Gravimeters (ICAG2009), with a relative uncertainty of 4.2 µGal. The measurement result was obtained at the height of 1300 mm, horizontally centered with respect to the vacuum chamber. For the BIPM site, a number of comparison reference values (CRVs) of past international comparisons of absolute g measurements (ICAGs) are available. These can be used to estimate the longterm stability of absolute g values at the BIPM Kibble balance. The measurement results show that the g value change was within 1 µGal over 8 years. This might be verified again in the future. In Kibble balances, the g value has to be transferred from the measurement position to the mass weighing position, and hence the relative gravity changes in space, i.e. the horizontal gravity gradient (HGG) and the vertical gravity gradient (VGG), need to be known. The HGG and VGG at the BIPM were determined by a combination of the relative gravity mapping and the evaluation of the instrumental selfattraction. A 3dimensional (3D) spatial array of g values was measured by a relative gravimeter inside the laboratory. Predictions were made from this 3D gravity map, which agreed to the offgrid experimental gravity measurement within 1 µGal. The selfattraction effect of the BIPM Kibble balance apparatus was modelled by using the mathematical analogy between the equations for the gravitational field and the electrical field based on an electrostatic finite element analysis (FEA). The gravitational field of individual construction segments of the BIPM Kibble balance was calculated and combined, yielding a total correction of (4.7 ± 0.5) µGal at the mean trajectory position.
The timevarying contribution of g, known as the Earth tides, polar motion, and atmospheric mass redistribution, was modelled by a software package based on localized parameter determination. The longterm measurement showed that this calculation can achieve an uncertainty of about 1 µGal. The present uncertainty on the determination of the gravitational acceleration g at the location of the test mass is 4.6 µGal, which is not a limiting factor for a determination of the Planck constant at the 10^{−8} level. For further details see:
Since 2005 the BIPM has been developing a Kibble balance as a means for the practical realization of the expected new definition of the kilogram in terms of the Planck constant. The BIPM Kibble balance operates using a An improved version of the apparatus is fully assembled and operational in air. The relative uncertainty on the Planck constant determination is currently For further details see: 


