What is the uncertainty of a ruler?
Ruler A has an uncertainty of ±0.1 cm, and Ruler B has an uncertainty of ±0.05 cm. Thus, (a) Ruler A can give the measurements 2.0 cm and 2.5 cm. (b) Ruler B can give the measurements 3.35 cm and 3.50 cm. Figure 2.2 Metric Rulers for Measuring Length On Ruler A, each division is 1 cm.
Consider as an example the micrometer caliper shown in Figure 3. The screw of this instrument has 2 threads per millimeter so that it advances .5 mm for each revolution. Since there are 50 divisions around the rim, the least count and the uncertainty of this instrument is 1/50 x 0.5 mm = 0.01 mm.
- Therefore the instrument uncertainty for the meter stick is ±0.1 cm. (± smallest division). Sometimes, one can estimate the instrument uncertainty by interpolation. The interpolation is usually estimated as a multiple of ½, 1/3 or 1/5, etc of the smallest division on the instrument.
- According to British Standards, correctly calibrated, used and maintained liquid-in-glass thermometers can achieve a measurement uncertainty of ±0.01 °C in the range 0 to 100 °C, and a larger uncertainty outside this range: ±0.05 °C up to 200 or down to −40 °C, ±0.2 °C up to 450 or down to −80 °C.
- AQA Science: Glossary - Zero Errors. Any indication that a measuring system gives a false reading when the true value of a measured quantity is zero, e.g. the needle on an ammeter failing to return to zero when no current flows. A zero error may result in a systematic uncertainty.
Updated: 28th November 2019