The vernier caliper, the vernier scale and measuring accuracy
The vernier caliper is a precise instrument for measuring a part’s outside and inside sizes and the depth of holes and grooves. Its sliding jaw carries an auxiliary scale called the vernier, which lets you measure to a tenth of a millimetre, that is, 0.1 mm accuracy. The vernier’s 19 mm scale is divided into 10 equal parts, so each division is 1.9 mm. Before measuring, close the jaws to check the instrument: the zero lines of both scales must coincide and the vernier’s tenth line must meet the nineteenth line of the main scale. When measuring, whole millimetres are counted on the main scale to the left of the vernier’s zero, and the tenths of a millimetre are given by the number of the vernier line that coincides with a main-scale line. For example, if the zero lies past 12 mm and vernier line 4 matches, the size is 12.4 mm. The tool is precise and costly, so before use wipe it with a soft cloth to remove oil and dust, do not rub it with abrasive paper, and do not leave it near a heater or in sunlight. The part must be dry and clean, the jaws not skewed and the locking screw tightened; read the scale looking straight at it. The jaw tips are sharp, so hold the instrument carefully and do not carry it with the tips pointing outward.
Measuring practice (supervised): each pair measures the thickness of a pencil, pen, nail and coin with a vernier caliper and records it in a table. One person uses an instrument at a time, wipes it afterwards and puts it in its case. Watch the sharp tips.