Controlling Torque:
When the current is removed from an instrument, the pointer of the instrument once deflected returns to the zero position due to the controlling torque. The controlling torque is the torque that brings the pointer back to its original position when there is no current flowing through the instrument. It is generated by a spring or some other mechanism in the instrument.

Explanation:
When current flows through an instrument, it creates a magnetic field that interacts with a permanent magnet or a coil inside the instrument. This interaction causes a torque to be exerted on the pointer, which causes it to move from its zero position.

When the current is removed, there is no longer any magnetic field to create a torque on the pointer. However, the controlling torque still acts on the pointer and brings it back to the zero position. The controlling torque counteracts the deflection torque and restores the pointer to its original position.

The controlling torque is usually generated by a spring mechanism in the instrument. The spring is designed to exert a torque that is proportional to the deflection of the pointer. As the pointer moves away from the zero position, the spring exerts a torque in the opposite direction, bringing the pointer back towards the zero position.

The controlling torque is designed to be stronger than the deflection torque caused by the current. This ensures that the pointer always returns to the zero position when the current is removed.

Summary:
The controlling torque is responsible for bringing the pointer of an instrument back to its zero position when the current is removed. It counteracts the deflection torque caused by the current and is usually generated by a spring mechanism. The controlling torque ensures that the instrument is reset to its original position when there is no current flowing through it.