A circuit for indicating the ratio of an unknown part value to a known part value, or the percentage error between an unknown part value and a known part value, or the direct value of an electrical part to be measured, has a first operational amplifier for developing a voltage proportional to the ratio of an unknown part value to a known part value and a second operational amplifier for subtracting a predetermined amount from said ratio voltage so that the resultant output from said second amplifier provides the desired percentage error of the unknown part value to that of known part value.