Development of a new apparatus for SI traceable small mass measurements using the voltage balance method at NMIJ

Toward establishing a measurement system for small masses and forces based on the new definition of the kilogram, a measurement apparatus that uses the voltage balance method was developed. The main components of the apparatus are a mass balance mechanism, an automatic mass translation system, and a capacitance actuator for balancing the gravity force acting on the small object and the electrostatic force. The electrostatic force is calibrated by using the electrical methods that are traceable to the electrical standards. Therefore, the voltage balance method realizes SI traceable and absolute measurements of small masses. To improve the method, new technical approaches, such as the use of a monolithic flexure hinge for the balance mechanism, a double equal capacitor system for the electrical control, and a new type of buckling spring, were implemented. To evaluate the performance of the apparatus, an experiment of the mass of a 500- $\mu \text{g}$ aluminum wire weight was conducted. A result of the weighing of the 500- $\mu \text{g}$ weight was $499.8~\mu \text{g}$ . The basic performance of the new apparatus for small mass measurements was confirmed.