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✔ Keep away from conductors (including people)
✔ Do not touch the jig (clip) during measurement
✔ Avoid vibrations
✔ Electrostatic shielding
Resistance is generally measured using a digital multimeter or ohmmeter. The principle of resistance measurement is explained in the column “Measuring Method & Handling Precautions for Low Resistance,” so this column shows practical precautions when measuring high resistance.
The measurement current used in the high resistance range of digital multimeters and resistance meters is very small, on the order of μA or nA. As a result, “external noise” and “leakage current” can affect the measurement system and cause errors.
External noise has an effect through physical phenomena such as “electrostatic coupling” and “electromagnetic induction.”
◇What is electrostatic coupling?
Electrostatic coupling in resistance measurements is a phenomenon in which floating capacitance occurs between the cable and conductor when there is a conductor near the measurement cable, generating an electromotive force. Humans are also conductors, so when the person taking the measurement approaches or moves away from the measurement system, the floating capacitance changes, causing variations in the measurement values.
◇What is electromagnetic induction?
The magnetic field in space acts on conductors (measurement cables and circuits inside measuring instruments), generating electromotive forces and currents. This phenomenon is well known as Fleming’s left-hand rule.
◇What is leakage current?
It is a minute current that flows through a path, even in a place that is supposed to be insulated. Ideally, all the measurement-current emitted by a measuring instrument should flow into the object being measured, but some are inevitably shunted as leakage current, resulting in error. Plastic and resin materials, which are usually treated as insulators, have a resistance of 109-1015 (Ω/cm) and allow a small amount of current to flow. The human body is also a conductor. The higher the resistance of the object being measured and the smaller the measurement current, the greater the impact of leakage current.
Example of leakage current: A small current (red arrow) flows through the hand.
The precautions to take when measuring resistance are explained in detail on measuring instrument manufacturer’s websites, but as an even easier solution, simply following the precautions mentioned at the beginning of this column can in some cases reduce the causes of error and improve the situation. Give it a try.
・Keep away from conductors (including people) = Prevent electrostatic coupling
・Do not touch the jig (clip) during measurement = Prevent leakage current
・Avoid vibrations = Prevent electromagnetic induction
・Electrostatic shielding and grounding = Prevent electrostatic coupling and electromagnetic induction
Example of shielding: When measuring, completely cover the resistor to be measured and connect it to earth.
Lineup of high resistance series
