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When dealing with breadboards, and electronic circuits in general, in my case finding the total (equivalent) resistance using an ohmmeter, what are the factors that make the experimental value not equal to the theoretical one?

Cables? The resistances themselves?

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The major factors are:

  1. Ohmmeter is not properly calibrated ("zeroed") to account for resistance of probe wires and clamp tips.

  2. Contact resistance between the probe tip and the circuit element due to the presence of solder resist residues, solder flux residues, or oxides/corrosion products.

  3. For old-style analog meters, low or defective batteries in the power supply or corrosion of battery contacts.

  4. Imprecision in the component itself (i.e., resistor labeled as 1K which measures out to 990 ohms) which is within the tolerance band for the component.

  5. For resistance measurements, the presence of electrolytic capacitors in the circuit measurement path which perturb the measurement by using the sense voltage injected by the meter to charge themselves.

  6. The presence of transistors in the measurement path, which introduce polarity-dependent current leakage during measurements.

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