STRAIN AMPLIFIER

PURPOSE:To avoid errors even when a drift voltage of a main body of a measuring apparatus is large, by providing a calibration means which is independent of the influences of both an offset voltage of an object to be measured and the drift voltage of the main body of the measuring apparatus. CONSTITUTION:When a power source 40 for driving a calibration means is turned on by a switch 43, the power voltage of a bridge circuit 12 becomes 0 volt, and at the same time the power voltage of an auto-balance circuit 50 is rendered 0 volt, and accordingly a canceling voltage from the auto-balance circuit 50 is also turned 0. In the state as above, if voltage is generated at an output terminal 70, the voltage can be regarded as a drift of a main body 100 of a measuring apparatus including a differential amplifier 20 and a gain switching amplifier 30. In a calculation circuit 60, the above-mentioned drift voltage is subtracted from an output voltage of the differential amplifier 20 in the balanced condition, so that an offset voltage of an object 10 to be measured and the drift of the main body 100 are compensated for.