In the constant voltage mode the two inputs to the differential amplifier are switched. The terminal that was connected to the input of the trans resistance amplifier in the constant current mode is now grounded. The other input of the differential amplifier that was connected to the voltage terminal of the unknown component is now connected to a point on the Z-magnitude range switch which is held at a constant potential. The voltage terminal of the unknown is connected to this same point of constant potential, or depending on the setting of the Z-magnitude range switch, to a decimal fraction of this voltage. In any case, the voltage across the unknown is held at a constant level. The current through the unknown is applied to the trans resistance amplifier which again produces an output voltage proportional to its input current.