Different types of lengths are measured by using different types of instruments. Lengths like the length of cloth or length of a line can be measured by using measuring tape, a metre scale or a foot rule. But these instruments cannot be used to measure the diameter of a metal sphere or a cylinder. To measure the diameter of a cylinder we can use paper strip method and wooden block method:
(i) The correct way to read a ruler is shown in the figure below. The eye must be positioned vertically above the mark to avoid error due to Parallax.
(ii) Paper strip method: Wind a strip of paper closely round the object once and prick the overlapping position with a pin (Shown in figure below).
A method for measuring diameter of a cylinder
Unwind the paper strip and measure the distance between the two pinholes. This measure is the measure of the circumference, since circumference = (
x diameter).
Hence now the diameter can be calculated.
(iii) Wooden block method: Place the sphere or the cylinder between two blocks in contact with a ruler as shown in figure below.
A simple method for measuring diameter of a sphere
Read the distance between the two blocks on the ruler accurately. (The line of sight should be vertical.)
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