How do you read a vernier caliper to the nearest 0.02 inch?

• A. Read the main scale to the mark just after the zero on the vernier, then read the vernier scale for the alignment line
• B. Read the main scale to the mark just before the zero on the vernier, then read the vernier scale alignment line, and add the vernier increment to the main scale reading
• C. Use only the main scale
• D. Use only the vernier scale

Answer: (B)

Reading a vernier caliper to the nearest 0.02 inch comes from combining two readings: the main scale for the whole inches and the vernier scale for the fractional part. First, look at the main scale and note the mark just before the zero of the sliding jaw. That gives the whole-number inches (and any fractions shown on the main scale up to that point). Next, find which line on the vernier scale lines up best with a line on the main scale. Each line on the vernier represents a 0.02 inch increment, so the aligned vernier line tells you the additional fraction to add. Add that vernier increment to the main-scale reading to get the measurement, accurate to 0.02 inch.

For example, if the main scale reads 2.00 inches just before the vernier zero, and the 6th line on the vernier aligns, you add 6 × 0.02 = 0.12 inches, giving 2.12 inches total. Using only the main scale wouldn’t achieve the 0.02 inch precision, and using only the vernier without tying it to the main scale wouldn’t provide the correct overall measurement.