The ballistic galvanometer is a type of galvanometer used to measure and detect small amounts of electric current. It is designed to have a high sensitivity and a very short response time. The key characteristic of a ballistic galvanometer is its ability to measure the charge passing through it in one direction only, without being affected by any subsequent changes in the current.

The term "ballistic" refers to the behavior of the galvanometer's pointer, which swings or oscillates rapidly in response to the passage of a charge. This swinging motion is due to the inertia of the pointer and is similar to the motion of a pendulum.

In order for the ballistic galvanometer to accurately measure the charge passing through it, it is important that the amplitude of the first swing of the pointer is very large. This is because the amplitude of the swing is directly proportional to the charge passing through the galvanometer. A larger swing indicates a larger charge, while a smaller swing indicates a smaller charge.

Therefore, the ballistic galvanometer is usually lightly damped to ensure that the amplitude of the first swing is very large. Damping refers to the resistance or friction that is applied to the motion of the pointer. If the galvanometer was heavily damped, the pointer would not swing as far and the amplitude of the first swing would be smaller. This would result in less accurate measurements of the charge.

By keeping the damping light, the ballistic galvanometer allows the pointer to oscillate freely and achieve a large amplitude of the first swing. This ensures that the galvanometer accurately measures the charge passing through it and provides reliable readings.

In summary, the reason why the ballistic galvanometer is usually lightly damped is because it allows for a large amplitude of the first swing of the pointer. This is crucial for accurate measurement of the charge passing through the galvanometer.