For lengths from about 10−5 m up to 102 m, length can be measured directly: a metre scale for 10−3 m to 1 m; a vernier caliper for finer work, down to about 10−4 m (least count 0.1 mm); and a screw gauge for the finest direct work, down to about 10−5 m (least count 0.01 mm), for objects up to roughly 50 mm.
Screw gauge. Works by magnifying a small linear motion into the easily-read circular motion of a screw. A reading combines the pitch scale reading (PSR) -- the last fully-visible mark on the linear scale -- with the head scale coincidence (HSC) -- the head-scale division exactly aligned with the reference line -- times the least count:
Reading=PSR+(HSC×least count).
Worked example: PSR =6 mm, HSC =40 divisions, least count =0.01 mm ⇒ reading =6+(40×0.01)=6.40 mm.
Vernier caliper. Measures the diameter or depth of a hole via the main scale reading (MSR) plus the vernier scale coincidence (VSC) -- the vernier division that lines up exactly with a main-scale division -- times the least count:
Reading=MSR+(VSC×least count).
Worked example: MSR =2.2 cm, VSC =4 divisions, least count =0.01 cm ⇒ reading =2.2+(4×0.01)=2.24 cm.
Zero error. If a reading is non-zero when the jaws/anvil are fully closed, this offset (positive or negative) must be added to, or subtracted from, every subsequent reading before it is trustworthy.
Comparing the two. The screw gauge's least count (0.01 mm) is finer than the vernier caliper's (0.1 mm) -- i.e. a smaller least count means a more precise instrument, because it can resolve a smaller change. A screw gauge with pitch 1 mm and 100 head-scale divisions has least count =1001 mm=0.01 mm, ten times finer than a vernier caliper whose sliding scale has 20 divisions over, say, 1 mm main-scale spacing (least count typically 0.1/20 or similar, always coarser than the screw gauge in this comparison) -- confirming the screw gauge is the more precise of the two for small-object measurements.