A moving-coil meter has 100 turns, coil length 40 mm, coil width 30 mm, and a uniform flux density B = 1 Wb/m^2. If the control (restoring) torque at full-scale is 240 × 10^-6 N·m, what is the full-scale current range of the meter?

Introduction / Context:
Permanent-magnet moving-coil (PMMC) instruments balance electromagnetic deflecting torque against a spring control torque. At full-scale deflection, the two torques are equal, which allows the full-scale current to be determined from geometry and magnetic field data. This is a standard single-step substitution problem in electrical measurements.

Given Data / Assumptions:

• Number of turns N = 100.
• Coil dimensions: length l = 0.04 m, width w = 0.03 m → area A = l * w = 0.0012 m^2.
• Flux density B = 1 Wb/m^2 (uniform in the air gap).
• Control torque at full scale T_c = 240 × 10^-6 N·m.
• At full-scale: deflecting torque T_d = control torque T_c.

Concept / Approach:
Deflecting torque for PMMC: T_d = N * B * I * A. Set T_d = T_c and solve for the full-scale current I_fs. This is simple algebra with consistent SI units.

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