Abstract: Methods and apparatus are disclosed for measuring an electrical current, particularly an AC electrical current, for example to measure a frequency response of a transformer. In a disclosed arrangement, a connection is made of a current input terminal for receiving a current to be measured to an input terminal of a differential amplifier via a portion of a resistive network. The resistive network holds the current input terminal at a ground voltage and the two input terminals of the amplifier at a common finite voltage above the ground voltage. The amplifier provides as output a measure of the current to be measured.