Which one of the statements given below is not correct ?

The statement in option 4 is incorrect because relative permeability is a dimensionless quantity. Relative permeability (μr) is defined as the ratio of the magnetic permeability of a medium (μ) to the permeability of free space (μ0). Since it is a ratio of two identical physical quantities, the units cancel out, making it a dimensionless scalar. Option 1 is correct as the magnetic meridian is the vertical plane passing through the magnetic axis of a freely suspended magnet. Option 2 is correct because the geographical meridian is the vertical plane passing through the Earth's axis of rotation [4]. Option 3 is a standard definition of permeability, representing the degree to which a medium allows magnetic field lines to penetrate it [3]. Therefore, the only false statement is that relative permeability is not dimensionless.

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 5: Earths Magnetic Field (Geomagnetic Field) > Geomagnetic Reversal > p. 74
2. [4] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 5: Earths Magnetic Field (Geomagnetic Field) > Geomagnetic Poles > p. 72
3. [3] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 5: Earths Magnetic Field (Geomagnetic Field) > Normal and Reversed Magnetic Field > p. 75