Force
In physics, a force is any interaction that, when unopposed, will change the motion of an object. A force can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a push or a pull. A force has both magnitude and direction, making it a vector quantity. It is measured in the SI unit of newtons and represented by the symbol F.
Bryan Kibble
In 1990, the Mark II watt balance was built, with a view to measuring the Planck constant accurately enough to redefine the S.I. unit of the kilogramme from fundamental constants. Such an instrument equalises one force with another, specifically the weight of a [one-kilogramme, typically] test mass and the force produced by an electric current in a moving coil in a magnetic field. The magnitude of the upward force on the coil is controlled by changes to the current. The power of the moving mass is equal to the product of the current and potential difference measured in the coil, hence the term 'watt' balance (as the S.I. unit of power is the watt (W)).