Identify the correct equation for controlling torque (T_C) in a spring-controlled moving iron instrument, where K is the spring constant and θ is the deflection angle.

• AT_C = K/θ
• BT_C = K + θ
• CT_C = K − θ
• DT_C = Kθ

Answer & step-by-step solution

Correct answer: D. T_C = Kθ

Quick explanation

A control spring produces restoring torque proportional to deflection: T_C = Kθ. At the final reading, deflecting torque equals controlling torque.

T_C = Kθ At equilibrium: T_D = T_C