Understanding PMMC Instrument Disadvantages

PMMC (Permanent Magnet Moving Coil) instruments are widely used for DC measurements due to their accuracy and linear scale. However, like any instrument, they have certain limitations or disadvantages.

Let's analyze the given options:

• The PMMC is very delicate instrument: PMMC instruments use fine wires for coils and delicate suspension (like springs or pivot and jewel bearing). This makes them susceptible to damage from vibration or shock, making them quite delicate.
• It has a high torque to weight ratio: A high torque-to-weight ratio is actually an advantage. It means the instrument is more sensitive and less affected by friction, contributing to better performance.
• It produces low losses due to hysteresis: PMMC instruments typically do not use iron in the moving coil itself, which minimizes hysteresis losses. This is an advantage, leading to more accurate readings.
• It does not have uniformly divide scale: This statement is incorrect. The deflecting torque in a PMMC instrument is directly proportional to the current flowing through the coil ($$\tau_d \propto I$$). Since the controlling torque is provided by springs ($$\tau_c \propto \theta$$), and in equilibrium $$\tau_d = \tau_c$$, the deflection angle $$\theta$$ is directly proportional to the current $$I$$. This results in a uniformly divided scale, which is a significant advantage.

Based on the analysis of the options, the statement that describes a disadvantage of PMMC instruments is that they are very delicate.