In the very ancient period of civilization human uses footsteps, foot and hand to measure the distance or length of a body. But those method creates various types of trouble. Suppose two carpenters want to build a table having four legs each of two hand long. For that they plan, each of them will make two leg for that table. After completing the table, it is found that the legs are not equal, which falls them in a trouble. To avoid this types of problem, people realize the necessity of standard scale all over the world. By this way we got several measurement scales, like light-year, mile, kilometer, meter, centimeter, millimeter, micrometer, Angstrom etc.
Centimeter scale uses all over the world; the minimum possible length measure by this scale accurately is one centimeter. Let us consider, somebody wants to measure the length of his pencil by using a centimeter scale and found the length is more than seven centimeter but less than eight centimeter as shown in figure-1. To measure accurately he may divide one centimeter by ten equal division, each division is called one millimeter. By this arrangement it is seen that the length of the pencil is seven centimeters and two millimeters. When magnified the scale (shown in figure-2) it is clearly shows that the length between 7.2cm and 7.3cm. If he divide one millimeter another ten division we measure the length more accurately, but still there is a limit of accuracy of 0.1mm. From the above discussion it can be understood that using small scale we measure more accurately; but he cannot measure the length of the pencil accurately.
In daily life it not essential to measure accurately; if we try to measure accurately in practical life some time it seems to be funny. Suppose somebody says the road distance between the zero point of Dhaka and Chittagong is 244km 62m 27cm 7mm …….. etc; he is not wrong but it seems to us that he is mad or insane. We usually say that the distance of Chittagong from Dhaka is 244km. But we cannot ignore 62 m when we measure fabric but may ignore 7mm. Again we cannot ignore 7mm when we measure the thickness of paper. To measure the size of the atom the accuracy level should be less than one Angstrom. That is why we uses the unit light-year to measure inter-galactic distance, kilometer for the distance between two places, meter to length of a building, millimeter for diameter of rod, pico-meter or angstrom for radius of a atom etc. As there is a limitation of accuracy in measurement, so there must be an error in every measurement. To know how much error we may tolerate it is necessary to know about accuracy and precision.
