What type of instrument is used for measuring very high values of resistance?

The megohmmeter is specifically designed for measuring high resistance values, typically in the range of megohms. This device applies a high voltage to the resistance being measured, which helps to overcome resistance effects that could interfere with measurements made with standard multimeters. Due to its capability to produce a higher voltage than typical measuring instruments, the megohmmeter is ideal for testing insulation resistance in electrical systems and ensuring that insulating materials are effective.

In contrast, while a multimeter can measure resistance, it is limited in the range it can effectively test, generally up to a few megaohms, making it unsuitable for extremely high resistance values. An oscilloscope is primarily used for visualizing electrical signals over time and does not measure resistance directly. A thermometer, on the other hand, is designed to measure temperature, which is unrelated to resistance measurement. These distinctions clarify why the megohmmeter is the correct choice for this question.