The magnetic compass gives directly the magnetic bearings of lines. The bearings may either be measured in the W С.В. system or in the Q.B system depending upon the form of the compass used.
The bearings so measured are entirely independent on any other measurement.
The general principle of all magnetic com pass depends upon the fact that if a long, narrow strip of steel or iron is magnetized, and is suitably suspended or pivoted about a point near its center so that can oscillate freely about the vertical axis, it will tend to establish itself in the magnetic meridian at the place of observation.
The most essential features of 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.
Prismatic compass is the most convenient and portable form of the magnetic compass either be used as a hand instrument or can be fitted on a tripod.
The magnetic needle is attached to the circular ring or compass card made up of aluminum, a non-magnetic substance. When the needle is on the pivot, it will orient itself in the magnetic meridian and, therefore, the N and S ends of the ring will be in this direction. The line of sight is defined by the object vane and the eye slit, both attached to the compass box. The object vane consists of a vertical hair attached to a suitable frame while the eye slit consists of a vertical slit cut into the upper assembly of the prism unit, both being hinged to the box. When an object is sighted, the sight vanes will rotate with respect to the NS end of ring through an angle which the line makes with the magnetic meridian.
The graduated ring is directly attached to the box, and not with a needle. The edge bar needles freely over the pivot. Thus, the graduated card or ring is not oriented in the magnetic meridian, as was the case in the prismatic compass. The object vane is similar to that of the prismatic compass. The eye vane consists of a simple metal vane with a fine slit. Since no prism is provided, the object is to be sighted first with the object and eye vanes and the reading is then taken against the North end of the needle, by looking vertically through the top glass. When the line of sight is in the magnetic meridian, over the 0° N and 0° S graduations of the graduated card. The card is graduated in quadrant system having 0° at N and S ends and 90″ at East and Went ends.
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