The magnetic field lines prefer to pass through iron than because the permeability of iron is much larger than air.

Phosphor - bronze wire is used as the suspension wire because the couple per unit twist is very small.

The galvanometer is said to be sensitive if it shows large scale deflection even though a small current is passed through it or a small voltage is applied across it.

One ampere is defined as that constant current when it is passed through each of the two infinitely long parallel straight conductors kept side by side parallely at a distance of one meter apart in vacuum causes each conductor to experience a force of 2 x 10^-7 newton per meter length of conductor.

When an electric charge q is kept at rest in a magnetic field, no force acts on it. At the same time, if the charge moves in the magnetic field, it experiences a force. This force is different from Coulomb's force. This force is known as magnetic Lorentz force
\(\vec { F } =q(\vec { v } \times \vec { B } )\)
In general, if the charge is moving in both the electric and magnetic fields, the total force experienced by the charge is given by \(\vec { F } =q(\vec { v } \times \vec { B } )q\vec { E } \) . It is known as Lorentz force.

The significance of hysteresis loop is that it provides information such as retentivity, coercivity, permeability, susceptibility and energy loss during one cycle of magnetisation for each ferromagnetic material. Therefore, the study of hysteresis loop will help us in selecting proper and suitable material for a given purpose.

The angle between magnetic meridian at a point and geographical meridian is called the declination or magnetic declination (D).

The straight line which connects magnetic poles of Earth is known as magnetic axis. A vertical plane passing through magnetic axis is called magnetic meridian and a great circle perpendicular to Earth's magnetic axis is called magnetic equator.

Hysteresis is the phenomenon of lagging of magnetic induction behind the magnetising field.

Biot-Savart's law states that, the magnitude of magnetic field \(d\vec { B } \) at a point P at a distance of r from the small elemental length taken on a conductor carrying current varies
(i) directly as the strength of the current I
(ii) directly as the magnitude of the length of element \(\vec { dl } \)
(iii) directly as the sine of the angle θ between \(\vec { dl } \) and \(\hat { r } \).
(iv) inversely as the square of the distance r between the point P and length of element \(\vec { dl } \).
\(d\vec { B } =\frac { { \mu }_{ 0 } }{ 4\pi } \frac { I\vec { dl } \times \hat { r } }{ { r }^{ 2 } } \)

Magnetic susceptibility is defined as the ratio of the intensity of magnetisation (\(\vec { M } \)) induced in the material due to the magnetising field |\(\vec H\)|.
\( \chi _{ m }=\frac { |\vec { M } | }{ |\vec { H } | } \).

Magnetic flux is defined as the number of magnetic field lines crossing per unit area kept normal to the direction of lines of force.

(i) When a substance is placed in a uniform magnetising field, the substance gets magnetised.
(ii) The total magnetic field inside the specimen is equal to the sum of the magnetic field produced in vacuum due to the magnetising field and the magnetic field due to the induced magnetism of the substance.

It is defined as the deflection produced per unit voltage applied across the galvanometer.
V_s =\(\frac { \theta }{ V } \)
V_s = \(\frac { \theta }{ IR_{ g } } =\frac { NAB }{ KR_{ g } } =V_{ s }=\frac { 1 }{ GR_{ g } } =\frac { { I }_{ s } }{ { R }_{ g } } \)