Uniform Scale Identification

The question asks to identify the control mechanism that results in a uniform scale in measuring instruments. A uniform scale means the divisions are equally spaced, implying a linear relationship between the measured quantity and the instrument's deflection.

Control Mechanisms and Scale Uniformity

Let's analyze the options based on how they provide controlling torque ($\tau_c$) and its relationship with the deflection angle ($\theta$):

• Spring Control: The controlling torque provided by a spring is directly proportional to the angle of deflection. Mathematically, $\tau_c \propto \theta$. If the deflecting torque ($\tau_d$) is also proportional to the quantity being measured (e.g., current $I$), then at equilibrium ($\tau_d = \tau_c$), the deflection angle $\theta$ is directly proportional to the measured quantity ($I \propto \theta$). This results in a uniform scale.
• Gravity Control: This method uses gravity. The controlling torque is proportional to the sine of the deflection angle, $\tau_c \propto \sin\theta$. This leads to a non-linear relationship between the measured quantity and deflection, resulting in a non-uniform scale, particularly compressed at the beginning.
• Air Damping & Eddy Current Damping: These methods are primarily used for damping oscillations to bring the pointer quickly to its final position. They do not provide the necessary controlling torque to determine the scale's linearity. Scale linearity depends on the *control* mechanism, not the *damping* mechanism.

Conclusion on Uniform Scale

Based on the analysis, the spring control mechanism provides a controlling torque proportional to deflection, which is essential for achieving a uniform scale.

Therefore, Spring Control is the mechanism that has a uniform scale.