In a prismatic compass,
- AMagnetic needle moves with box
- BMagnetic needle and graduated circle do not move with boxCorrect
- CLine of sight does not move with box
- DGraduated circle is fixed to the box and the magnetic needle always remains in the N-S direction
Solution
Concept
Working of a prismatic compass
Step-by-step solution
1. In a prismatic compass the graduated aluminium ring is attached to the magnetic needle. 2. The needle and the ring remain fixed in the magnetic meridian when the box is turned. 3. The line of sight and the box move.
Explanation
In the prismatic compass the graduated circle is attached to the needle, so the needle and the circle always point to magnetic north and do not rotate with the box. The line of sight moves with the box and the reading is taken against it.
Shortcut
Prismatic compass: ring + needle fixed to magnetic N; box + sight vane move. Surveyor's compass: ring fixed to box.
Common mistake
Choosing (D), the surveyor's compass arrangement.
Correct answer: B) Magnetic needle and graduated circle do not move with box