NDA (I) 2017 General Ability Test · Question 53 of 149

Correct — A, Magnetic meridian. The magnetic meridian at a place is the vertical plane that contains the earth's magnetic field there, and equivalently the plane in which a freely suspended magnetic needle settles. Where that plane meets the ground it marks the magnetic north-south line, and the horizontal component of the earth's field points exactly along that line. This is what an ordinary compass responds to — its needle is pivoted so that it can turn only in a horizontal plane, so the only part of the earth's field that can act on it is the horizontal component, and the needle comes to rest lying in the magnetic meridian. UPSC's official key for this item marks (a), so the magnetic meridian is the answer to carry into the exam. Note that the phrase 'becomes horizontal' also has a second, very common reading, which is set out in the background below.

• (b)Magnetic equator — This is the strongest rival and it is worth being honest about it — the magnetic equator, also called the aclinic line, is the line along which the angle of dip is zero, so the earth's total field there has no vertical part at all and lies wholly in the horizontal direction. Many candidates and several solution books chose it for exactly that reason. The official UPSC key for this paper nevertheless marks (a), so answer to the key here and carry both facts forward — dip is zero at the magnetic equator, and the field at any place lies in the vertical plane called the magnetic meridian.
• (c)Geographical pole — The geographic poles are defined by the earth's axis of rotation, not by its magnetism, and the magnetic poles do not sit on them. Near a magnetic pole the field is very nearly vertical, with a dip approaching ninety degrees, which is the opposite of horizontal.
• (d)Tropic of Cancer — The Tropic of Cancer is an astronomical line, fixed by the northernmost latitude at which the Sun can be overhead. It has nothing to do with the earth's magnetism and no particular value of dip attaches to it.

Three quantities describe the earth's field at any place. The magnetic meridian is the vertical plane containing the field there; the angle of declination is the angle between that plane and the geographic meridian; and the angle of dip is the angle the total field makes with the horizontal. The field is fully horizontal only where the dip is zero, which happens along the magnetic equator, and it is nearly vertical near the magnetic poles. The horizontal component is what a compass needle feels, and it always points along the magnetic meridian.

This item has a genuine tension in it and students deserve to see it plainly. Read as 'where does the field have no vertical component', the physics answer is the magnetic equator, where the dip angle is zero. Read as 'in which plane does the field of the earth act on a horizontally pivoted needle', the answer is the magnetic meridian, which is what the official key marks. The key decides the mark, so option (a) is the one to reproduce; the physics of the magnetic equator is the one to remember for any statement-based question about dip. India has a direct hook for the second fact — the rocket launching station at Thumba in Kerala was sited there precisely because the magnetic equator passes over it, and UPSC has tested that in 2005.

• The magnetic meridian at a place is the vertical plane containing the earth's magnetic field there, and the horizontal component of the field lies along it.
• The angle of dip is zero along the magnetic equator, so the earth's total field there has no vertical component.
• The angle of dip approaches ninety degrees near the magnetic poles, where the field is almost vertical.
• The angle between the magnetic meridian and the geographic meridian at a place is called the magnetic declination.
• Thumba in Kerala was chosen for India's equatorial rocket launching station because the magnetic equator passes over it.

• Assuming the magnetic poles sit on the geographic poles — they do not, and the offset is measured by the declination.
• Confusing the magnetic equator, a line on the earth's surface, with the magnetic meridian, a vertical plane at a place.
• Forgetting that a compass responds only to the horizontal component of the earth's field.

NDA and UPSC both like the elements of the earth's magnetism — expect a direct question on where the dip is zero or ninety degrees, or a statement set mixing declination, dip and the Thumba fact.

• practice — not a real PYQ

  The angle of dip at the magnetic equator is

  • (a)0 degree
  • (b)30 degrees
  • (c)45 degrees
  • (d)90 degrees

  Answer(a) 0 degree — along the magnetic equator the earth's field has no vertical component, so the dip needle rests horizontally.

• practice — not a real PYQ

  The angle between the magnetic meridian and the geographic meridian at a place is called the

  • (a)angle of dip
  • (b)magnetic declination
  • (c)magnetic inclination
  • (d)solar declination

  Answer(b) magnetic declination — it measures how far magnetic north lies from true north at that place.