PURPOSE: To detect a current, which flows through an inductance composed of operational amplifiers, a resistance and a capacitor, as a voltage, by calculating the current through electronic arithmetic.

CONSTITUTION: An inductance circuit consists of operational amplifiers A1∼ A3, a resistance R and a capacitor C, and impedance Z between an input terminal 1 and an output terminal 2 is equal to SCR2, simulating inductance L=CR2. To this inductance circuit, a circuit consisting of an operational amplifier A4 and resistances R1∼R4 is added. Then, R(i1-i2)=-5/2.e2+e5 and when R4/ R3=R2/(R1+R2)=1/3.5, the output voltage e0 of the operational amplifier A4 is -5/2e2+e5, thereby calculating a current i1-i2 flowing through the inductance L=CR2.