^Apparent dip

Let respectively be the vertical component, horizontal component & dip angle in a vertical plane inclined at some angle say α to magnetic meridian, then .

On dividing we wet tanδ^/ = tanδ secα

From above relation we can write

• As sec α > 1, thus for any vertical plane inclined at some angle say α to magnetic meridian dip angle is greater than its value in magnetic meridian i.e. δ^/ > δ.
• δ^/ = 90⁰ if α = 90⁰e. in a plane perpendicular to magnetic meriadian dip needle will be vertical.

In a similar way it can be proved that if δ₁ and δ₂ be the angles of dip observed in two vertical planes at right angles to each other and δ is the true angle of dip, then cot² δ₁ + cot² δ₂ = cot² δ.

Facts

1. → Induced magnetic field or magnetic induction vector or magnetic flux density.
2. SI unit of : weber/meter² or tesla (T)
3. CGS unit: maxwel/cm² or gauss (G)
4. 1 T = 1 N A^-1 m ^{– 1} = Wb m^-2 = 10 ⁴ Gauss
5. _μ0 = 4 π x 10 ^{– 7} A^{– 1} Tm = 12.75 x 10 ^{– 7} A^{– 1} Tm (known as the permeability of air or vacuum).
6. 1 A^{– 1} Tm = 1 H m^–1 = 1 Wb A^{– 1} m^{– 1}
7. = 10 ^{– 7} A^{– 1} Tm

^Least count, accuracy & significant digits

Suppose a rod is measured by a metre stick, and is estimated to lie between 1.6 and 1.8 m, then its length can be written as 1.7 m. It contains two significant digits, of which we are perfectly sure of the position of 1, but slightly doubtful regarding the position of 7.

Now, suppose the same rod is measured by a metre scale graduated to centimetres and is estimated to lie between 173.2 cm & 173.4 cm then its length can be written as 173.3 cm or 1.733 m. Now, the number of significant digits are four namely, 1, 7, 3 and 3 and hence there is an increment in the accuracy of the value. Smaller the least count of a measuring instrument, more will be its accuracy in measurement and accordingly more will be the number of significant digits.