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The pointer of an indicating instrument should be
Very light
Very heavy
Either (A) or (B)
None
Very light
Quick Summary: The pointer of an indicating instrument must be very light to ensure high sensitivity and minimize the inertia of the moving system. A lighter pointer requires less deflecting torque to overcome the inertia and friction, leading to faster response times and reduced wear on the jewel bearings.
The pointer of an indicating instrument must be very light to ensure high sensitivity and minimize the inertia of the moving system. A lighter pointer requires less deflecting torque to overcome the inertia and friction, leading to faster response times and reduced wear on the jewel bearings.
J=∑mr2 — Moment of inertia of the moving system
Td=Jdt2d2θ+Ddtdθ+Kθ — Equation of motion for a measuring instrument
In an indicating instrument, the deflecting torque (Td) must overcome the inertia (J) and the damping torque (Tc). If the pointer is heavy, the moment of inertia J increases, which is governed by J=∑mr2. By keeping the mass (m) very low, the system achieves a faster settling time and higher sensitivity, allowing the instrument to track rapid changes in the measured quantity accurately.
Minimizing pointer mass reduces the mechanical time constant of the instrument.
A heavy pointer increases the load on the pivot bearings, leading to increased friction error.
Aluminum is commonly used for pointers due to its high strength-to-weight ratio.
Lower inertia prevents overshoot and oscillations around the final steady-state value.
High sensitivity
Fast dynamic response
Reduced bearing friction
Material must be chosen carefully to maintain rigidity
Fragility due to extremely thin construction
Moving coil (PMMC) voltmeters and ammeters
Moving iron instruments
Analog multimeters
Most pointer materials are made of thin-walled aluminum tubing or flattened aluminum strips to maintain structural integrity while minimizing mass.
Option B is incorrect because a heavy pointer would create excessive inertia, leading to sluggish response, high settling time, and significant mechanical wear on the pivots.
A is correct — The pointer must be very light to minimize inertia, thereby allowing for faster response, higher sensitivity, and reduced wear on the instrument's bearings.
Always remember that in measuring instruments, any mechanical component attached to the moving system should be designed for minimum weight to preserve accuracy and avoid transient errors.