Prismatic compass is the most convenient and portable form of magnetic compass which can either be used as a hand instrument or can be fitted on a tripod.
- It consists of a circular box of about 100 mm in diameter. A broad magnetic needle balanced on a hard steel pointed pivot.
- An aluminium ring graduated in degrees and half half degrees is attached to the needle.
- The observations are seen in clockwise from 0 to 360º.
- The graduations are inverted and they are viewed through prism.
- When the needle is balanced on the pivot, it orients itself in the magnetic meridian.
- The objective vane carries a vertical hair of fine silk thread.
- The sight vane consists of vertical slit cut in to the upper graduated assembly of the prism.
- Sun glasses are provided on the prism to sight luminous objects.
- Breaking pin provided at the base of the object vane is used to dampen the oscillations of the needle to facilitate the readings.
- Prismatic compass reads whole circle bearing of the lines of objects directly
Adjustment of Prismatic compass:
The following are the adjustments usually necessary in the prismatic compass.
a) Station or Temporary Adjustments:
- Focusing the prism.
b) Permanent adjustments:
the permanent adjustments are those adjustments which are done only when the fundamental relations between the parts are disturbed. They are, therefore, not required to be repeated at every set up of the instrument. These consist of:
- Adjustment of levels.
- Adjustment of sight vanes.
- Adjustment of needle.
- Adjustment of pivot point.
Temporary adjustments are those adjustments which have to be made at every set up of the instrument. They comprise the following
Centering is the process of keeping the instrument exactly over the station. Ordinary prismatic compass is not provided with fine centering device as is generally fitted to engineer’s theodolite. The centering is invariably done by adjusting or manipulating the legs of the tripod. A plumb bob may be used to judge the centring and if it is not available, it may be judged by dropping a pebble from the centre of the bottom of the instrument.
If the instrument is a hand instrument, it must be held in hand in such a way that graduated disc is swinging freely and appears to be level as judged from the top edge of the case.Generally,a tripod is provided with ball and socket arrangement with the help of which the top of the box can be leveled.
3) Focusing the Prism:
The prism attachment is slided up or down for focusing till the readings are seen to be sharp and clear.