Measurements and measurement accuracy
On the scale of a measuring instrument there are small unlabelled marks between the labelled ones. The value between two neighbouring marks is called the scale division, and it gives the accuracy of the instrument. To find it, divide the difference of two neighbouring labelled marks by the number of intervals between them. For example, if there are 10 equal intervals between 1 cm and 2 cm on a ruler, the division is (2 − 1) : 10 = 0.1 cm. The largest value the instrument can measure is its measuring limit. Every measurement has some uncertainty — an error; it is usually taken as half of the scale division (0.05 cm for a ruler with 0.1 cm division). To reduce the error, a quantity is measured several times and the average is calculated: the sum of all results divided by the number of measurements. The result is written as A = a ± Δa, where a is the result and Δa is the error.
“Measurement contest”: each group measures the length of a desk three times with a ruler and writes the results and the average on the board. Whose results are closest to each other and why? Discuss.