Deflecting Torque Equation and its Components

The question asks about the meaning of the variable "B" in the given deflecting torque equation: Deflecting torque = N.B.A.I. This equation is fundamental in understanding the operation of many electrical measuring instruments, particularly moving coil instruments.

Let's break down the equation: \( \text{Deflecting torque} = N \cdot B \cdot A \cdot I \)

In this formula, each letter represents a specific physical quantity:

• \( \text{N} \) represents the Number of turns in the coil. A higher number of turns generally results in a stronger torque for the same magnetic field and current.
• \( \text{B} \) represents the magnetic field strength or Flux Density in the gap where the coil moves. This is typically provided by a permanent magnet or electromagnet. A stronger magnetic field produces a larger deflecting torque.
• \( \text{A} \) represents the Area of the coil. The effective area of the coil that interacts with the magnetic field influences the torque produced.
• \( \text{I} \) represents the Current flowing through the coil. This is the quantity being measured by the instrument. The torque is directly proportional to the current.

The deflecting torque is the force that causes the pointer of the instrument to move away from its zero position, against the restraining torque provided by springs.

Identifying "B" in the Equation

Based on the standard definition of this equation in physics and electrical engineering, the variable "B" consistently represents the magnetic field strength, which is also known as magnetic flux density. It is a measure of the density of magnetic field lines passing through a unit area perpendicular to the field.

Analyzing the Options

Let's look at the given options in the context of our understanding of the deflecting torque equation:

1. Constant: While some variables might be kept constant in a specific design, "B" itself represents a physical property (magnetic field density) and isn't just a generic constant in the equation's definition.
2. Flux Density: This term accurately describes what "B" represents in the equation. It is the magnetic flux per unit area.
3. Current flowing throughout the meter: This is represented by "I" in the equation, not "B".
4. Number of turns in the coil: This is represented by "N" in the equation, not "B".

Therefore, "B" in the equation Deflecting torque = N.B.A.I specifically indicates Flux Density.