Electrical resistance is the easiest electrical property to measure precisely over a wide range at moderate cost. This is a simple design application for digital multimeter costing a few tens of dollars can measure resistances in the range 10 ohm to 10 megohm with a precision of about 1% using a two-wire technique.
Providing RL1 and RL2 are well-matched, three-wire techniques can be used. Circuit 2 employs two matched current sources, I1 and I2, to eliminate the effects of lead resistance. Circuit 3 is an AC-bridge that is in-balance when RX=RY. If a lock-in amplifier is used as a null-detector, determination of RX with an extremely low excitation current is possible.
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