"Jerky movement" refers to the typewriter carriage moving irregularly, skipping spaces, or jumping forward violently instead of moving one character at a time.
(i) Over-tension of the Mainspring: If the mainspring is wound too tightly, it exerts excessive force on the carriage. This pressure can cause the "dogs" (loose and rigid) to slip over the teeth of the escapement wheel, leading to a sudden, jerky jump.
(iii) Worn-out teeth of the Escapement Wheel: The escapement wheel relies on sharp, precise teeth to catch the dogs. If these teeth are worn out or chipped, the dogs cannot hold the wheel securely, causing the carriage to "skip" or move jerkily across the points of wear.
Why (ii) is incorrect: Low tension of the mainspring usually results in a sluggish or "sticky" carriage that fails to move at all, or moves too slowly to keep up with the typist. It does not typically cause the violent, erratic movement associated with being "jerky."
