Current Balance

(or ampere balance), a device used to reproduce the unit of electric current, the ampere. The design of the current balance is similar to that of an analytic balance, except that a current balance is made of nonmagnetic materials. The current is determined from the force of the electrodynamic interaction between two conductors in the form of coaxial single-layer solenoids, through which the same current is flowing. The force of interaction between the solenoids is counterbalanced by the weights, according to the equation , where I is the current, m is the mass of the weights, g is the acceleration of free fall, and k is a coefficient that depends on the dimensions of the solenoids and takes into account the properties of interaction of the solenoids compared to the interaction of rectilinear conductors. To counterbalance the weights in the absence of a current flow, a solenoid is suspended from the arm of the balance that carries the weights. This solenoid is identical to the movable solenoid, but no current flows through it.

In the USSR the current balance is used as the state standard for measurement of electric currents. The error of a current balance is about 0.001 percent.