Prismatic Compass
Given figure shows the cross-section of a typical prismatic compass [see described plate also].
A magnetic needle of broad form (1) is balanced on a hard and pointed steel pivot (2). The top of the pointed pivot is protected having agate cap (3).
1. Needle 10. Focussing stud
2. Pivot 11. Dark sunglasses
3. Agate cap 12. Box
4. Graduated disc 13. Glass cover
5. Slit metal frame 14. Lifting pin
6. Horse hair 15. Light spring
7. Mirror 16. Brake pin or knob
8. Reflecting prism having cap 17. Lifting lever
9. Eye vane 18. Support to fit on tripod
Prismatic compass
Prismatic compass
To the top of the needle an aluminium graduated disk (4) is fixed. When read from top Graduations are from zero to 360° in clockwise direction. The direction of north is treated as zero degrees, south as 180°, east as 90° and west as 270°. However, whereas taking the readings observations are at the other end of line of sight. Therefore, the readings are shifted by 180° and graduations are marked as shown in given figure. Because they are read through a prism Graduations are marked inverted.
(a) (b)
The line of sight which is consists of object unit and the reading unit. The Object unit consists of a slit metal frame (5) which is hinged to the box. In the centre with a horse hair or a fine wire or thread (6) the slit is provided. With a hinged mirror (7), the metal frame is provided which can be placed upward or downward on the frame. It can be slides along the frame. To view objects too high or too low the mirror can be adjusted from the position of compass. At diametrically opposite edge Reading unit is provided. With a sighting eye vane (9) it consists of a prism (8). On the graduation disk the prism magnifies the readings just below it. The prism is lowered or raised on the frame carrying it for focussing, and then fixed with the stud (10). Near the line of sight Dark sunglasses (11) provided can be interposed if the object to be sighted is bright (For example, sun).
The bottom of the box (12) which is approximate 85 mm to 110 mm supports the pivot of needle at its centre firmly. On the sides of the box the object vane and the prism are supported. With a glass (13) lid, the box is provided which protects the graduation disc at the same time permit the direct reading from the top. It presses a lifting pin (14) when the object vane is folded on the glass top which activates lifting lever (15) lifts the needle off the pivot. Therefore, it prevents undue wear of pivot point. Whereas taking reading, if graduation disc vibrates, with a spring (16) it can be dampened. A knob or brake pin (17) is provided for pressing spring on the box. When not in use prism may be folded over the edge of the box. Box is provided with a lid to close it while the compass is not in use. The box is provided having a socket to fit it on the top of a tripod.