Which device is used to measure very high resistances?

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Measuring very high resistances requires a tool that can apply a high test voltage and sense the tiny leakage current that flows through insulation. A megger is built for this task: it applies a substantial voltage across the material or insulation and measures the resulting current to calculate resistance in the megohm range. This makes it ideal for checking insulation resistance in cables, motors, transformers, and other installations where leakage resistances are values that ordinary meters can’t accurately read.

An ohmmeter is designed for low resistances, injecting a small current and reading voltage drop, so it’s not suited for insulation or very high-resistance measurements. An ammeter measures current, not resistance. A multimeter can measure resistance but typically only up to a few megohms and not at the high voltages or accuracy needed for insulation testing.