The pointer of an indicating instrument should be

Question

Examoogle · Accepted Answer

Very light

Explanation: 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.

Working Principle: In an indicating instrument, the deflecting torque ($T_d$) must overcome the inertia ($J$) and the damping torque ($T_c$). If the pointer is heavy, the moment of inertia $J$ increases, which is governed by $J = \sum mr^2$. 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.

Key Formulas: $J = \sum mr^2$ — Moment of inertia of the moving system; $T_d = J \frac{d^2	heta}{dt^2} + D \frac{d	heta}{dt} + K	heta$ — Equation of motion for a measuring instrument

Key Points: 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.

Advantages: High sensitivity; Fast dynamic response; Reduced bearing friction

Disadvantages: Material must be chosen carefully to maintain rigidity; Fragility due to extremely thin construction

Applications: Moving coil (PMMC) voltmeters and ammeters; Moving iron instruments; Analog multimeters

Additional Info: 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.

Answer: 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.

Exam Tip: 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.

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