The instrument used to measure high resistance is known as a Megger.

Megger:
- A Megger is a portable instrument used to measure the insulation resistance of electrical systems and equipment.
- It is specifically designed to measure high resistance values, typically in the range of megohms to gigohms.
- The term "Megger" is actually derived from the name of the company that first manufactured these instruments, which was the Electrical Power Measurement Company Limited.
- Meggers work on the principle of applying a high DC voltage to the insulation being tested and measuring the resulting current flow.
- They are commonly used in applications such as testing the insulation of cables, motors, generators, transformers, and other electrical equipment.

Working of a Megger:
- A Megger consists of a hand-cranked generator, a high-voltage power supply, and a measuring instrument.
- When the hand crank is turned, it generates a high DC voltage, typically 500 to 1000 volts, which is applied to the insulation being tested.
- The high voltage creates a potential difference across the insulation, causing current to flow through any leakage paths or defects in the insulation.
- The magnitude of the current flow is measured by the instrument and displayed as a resistance value.
- The resistance reading obtained indicates the quality of the insulation. Higher resistance values indicate good insulation, while lower values indicate possible faults or moisture in the insulation.

Advantages of using a Megger:
- Meggers are portable and easy to use, making them suitable for field testing.
- They provide a quick and reliable method of assessing the insulation resistance of electrical systems.
- Meggers can detect insulation problems before they lead to equipment failure, helping to prevent costly repairs and downtime.
- They are non-destructive and do not cause any harm to the insulation being tested.
- Meggers are designed to withstand high voltages and provide accurate readings even in harsh electrical environments.

In conclusion, a Megger is the instrument of choice for measuring high resistance values. It is a portable and reliable instrument that is widely used in the electrical industry for insulation testing.