Theory of magnetic compass & requirements of a magnetic needle -pdf

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Theory of magnetic compass & requirements of a magnetic needle -pdf
Theory of magnetic compass & requirements of a magnetic needle -pdf


The general principle of all magnetic compass depends upon the fact that if a long, narrow strip of steel or iron is magnetised, and is suitably suspended or pivoted about a point near its centre so that it can oscillate freely about the vertical axis, it will tend to establish itself in the magnetic meridian at the place of observation.

Magnetic compass gives directly the magnetic bearings of lines. The bearings may either be measured in the W.C.B. system or in Q.B. system depending upon the form of the compass used. The bearings so measured are entirely independent on any other measurement. 

The most essential features of a magnetic compass are: 

  • Magnetic needle, to establish the magnetic meridian.
  • A line of sight, to sight the other end of the line. 
  • A graduated circle, either attached to the box or to the needle, to read the directions of the lines.
  • A compass box to house the above parts. 

In addition, a tripod or suitable stand can be used to support the box.

The various compasses exhibiting the above features are: 

  1. Surveyor's compass
  2. Prismatic compass
  3. Transit or Level Compass. 

Earth's Magnetic Field and Dip

The earth acts as a powerful magnet and like any magnet, forms a field of magnetic force which exerts a directive influence on a magnetised bar of steel or iron. If any slender symmetrical bar magnet is freely suspended at its centre of gravity so that it is free to turn in azimuth, it will align itself in a position parallel to the lines of magnetic force of the earth at that point.

Theory of magnetic compass- c.s of Earth's magnetic field
Theory of magnetic compass- c.s of Earth's magnetic field

The lines of force of earth's magnetic field run generally from South to North. Near the equator, they are parallel to the earth's surface. The horizontal projections of the lines of force define the magnetic meridian. The angle which these lines of force make with the surface of the earth is called the angle of dip or simply the dip of the needle. In elevation. these lines of force (ie. the North end of the needle), are inclined downward towards the north in the Northern hemisphere and downward towards South in Southern hemi- sphere. At a place near 70° North latitude and 96 West longitude, it will dip 90°. This area is called North magnetic pole. A similar area in Southern hemisphere: is called the South magnetic pole. At any other place, the magnetic needle will not point towards the North magnetic pole, but it will take a direction and dip in accordance with the lines of force at the point. since the lines of force are parallel to the surface of the earth only at equator, the dip of the needle will be zero at equator and the needle will remain horizontal. At any other place, one end of the needle will dip downwards. By suitably weighting the high end of the needle may be brought to a horizontal position.

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The Magnetic needle 

The compass needle is made of a slender symmetrical bar of magnetised steel or iron. It is hung from a conical jewel bearing supported on a sharp, hardened steel pivot. Before magnetisation, the needle is free to rotate both vertically and horizontally and does not tend to move away from any direction in which it is originally pointed. When it is magnetised, it will dip downwards and take a definite direction of magnetic meridian. A small coil of brass wire is wrapped around it to balance the force tending to make the needle dip. The position of the coil is adjustable for the dip in the locality where the compass is to be used. 

requirements of a magnetic needle
requirements of a magnetic needle

Above image shows a typical needle in section, which can either be a "broad needle" weight or "edge bar" needle type.

The pivot is a sharp and hard point and the slightest jar will break its tip or make it blunt. A lever arrangement is usually provided for lifting the needle off its bearing when not in use, so as to prevent unnecessary wear of the bearing with consequent increase in friction. 

Requirements of a Magnetic Needle

The following are the principal requirements of a magnetic needle: 

(1) The needle should be straight and symmetrical and the magnetic axis of the needle should coincide with the geometrical axis. If not, the bearing reading will not be with reference to the magnetic axis, and, therefore, will be wrong. However, the included angles calculated from the observed bearings will be correct.

(2) The needle should be sensitive. It may loose its sensitivity due to 

  • loss of  polarity, 
  • wear of the pivot. If the polarity has been lost, the needle should be remagnetised. The pivot can either be sharpened with the help of very fine oil stone or it may be completely replaced. Suitable arrangement should be provided to lift the needle off the pivot when not in use.

(3) The ends of the needle should lie in the same horizontal and vertical planes as those of the pivot point. If the ends are not in the same horizontal plane as that of the pivot point, they will be found to quiver when the needle swings, thus causing inconvenience in reading.

(4) For stability, the centre of the gravity of the needle should be as far below the pivot as possible.

In addition to the above requirements of the needle, the compass box along with other accessories should be of non-magnetic substance so that needle is uninfluenced by all other attractive forces except that of the earth's.

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