A differentiating interferometer is proposed for measuring time varying signals. The system comprising a modified fiber ring resonator was implemented to measure electrical current. The ring interferometer consists of a length of fiber between one of the input and output ports of a directional coupler. This length of fiber forms a delay line resulting in a constant delay time. The other coupler output port is connected to the sensing element and a reflector. By virtue of the time delay the system detects the derivative of the unknown perturbation signal at the common input/output port of the interferometer. This configuration ensures a zero path length imbalance. Electrical current measurements are achieved by means of a current transformer terminated with a resistive load, which drives a piezoelectric phase modulator. The interferometer can be optically biased by a polarization controller to measure differential phase linearly over the range -?/2 to ?/2 rad. An integration function yields the unknown electrical current. By employing a directional coupler in the system another reflective interferometer arm can be formed for multiplexed operation. The introduction of frequency domain demultiplexing allows interrogation of the sensors in each of these arms. Different phase generated carriers are applied to each arm with two additional piezoelectric phase modulators. Synchronous and quadrature phase detection yield the two current signals. Results of current measurements between 9 A and 300 A on two 50 Hz phases are discussed for the optically biased and phase generated carrier configurations.