Here is a current source you can build for resistance measurement. When the current is held constant, you know as per Ohm’s Law the Voltage across Resistor is proportional to Resistance value.

The supply is +12 and -12, The total voltage across R6 + R7 is 24V. Then 24V / 120K = 0.2mA. The voltage across R6 is (10K * 0.2mA) = 2V. The same is reflected across R5 in this feedback configuration. That means Q3 is a 2V / 1K = 2mA source. If my calculations are right.

There are sources of errors in this circuit. The temperature variation of all resistor values, which is 100ppm for general calculations in 1% MFR. Let us assume you use OP07 which is close to an ideal opamp, but for this application it is not needed. The second error is Ib, the base current of Q3 which may be 0.2mA / Hfe(200) = ~ 1 uA. Then the variation of Hfe, Vcc and Vdd w.r.t. Temperature, should not be overlooked. Use LM7812 and LM7912.

So you see, design knowing that all these components are not ideal. Leakage currents, Humidity, EMI, Stray Capacitance and Inductance and much more. It is just like, even when the motor is fixed firmly on the machine, some parts Vibrate and create a Noise due to Mechanical Resonance. So Build and evaluate your design in the real environment, to learn.

Discover how resistors are color coded – Interactive Java Resistors Tutorial.

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