The correct answer is: C. coil
A megohmmeter, also known as a megger, is an instrument used to measure high insulation resistance. It consists of a hand-cranked generator that produces a high voltage, which is applied to the insulation being tested. The insulation resistance is then measured by the megohmmeter.
The controlling torque in a megohmmeter is provided by a coil. The coil is connected to the generator and the insulation being tested. When the generator is cranked, the coil produces a magnetic field. This magnetic field interacts with the current flowing through the insulation, creating a torque that turns the pointer on the megohmmeter.
The other options are incorrect because they do not provide the controlling torque in a megohmmeter.
- Option A: A spring is not used to provide the controlling torque in a megohmmeter. The spring is used to keep the pointer on the megohmmeter in the zero position when the generator is not cranked.
- Option B: Gravity is not used to provide the controlling torque in a megohmmeter. The megohmmeter is a handheld instrument and is not affected by gravity.
- Option D: Eddy currents are not used to provide the controlling torque in a megohmmeter. Eddy currents are induced currents that flow in a conductor when a magnetic field is applied. They are not used in megohmmeters.