High precision balance for measuring quantity of dispensed fluid as a new calibration standard for the Becquerel

The 2019 redefinition of the kilogram has fundamentally transformed mass metrology, enabling direct traceability of the smallest masses to the SI system. Conventional methods such as mass subdivision disrupt this metrological chain and increase relative uncertainty in mass. This dissertation presents a solution to this problem through the development of an SI-traceable electrostatic force balance. As a central component of the "True Becquerel" project at the National Institute of Standards and Technology (NIST) in the USA, the system was designed to determine the mass of dispensed radionuclide solutions in the range of 1 to 5 mg with a relative uncertainty of less than 1 × 10−3. This work demonstrates the potential of electrostatic balances as primary mass standards in the milligram range and underscores their essential role in the future development of the SI system.