Use of Metre Scale:
For day-to-day work, a metre scale is used. It is divided into 100 parts called centimetres. Each centimetre is further divided into ten equal parts called millimeters. A metre scale is usually made from seasoned wood, plastic, or metal. For convenience, half-metre and still smaller scales are available in the market.
In order to measure the length of an object, say a blade, place it along one edge of the scale in such a manner that it’s one end coincides with any whole division and then read the scale. While reading the scale, the line of sight must be kept vertical. The below figure illustrates three positions of the eye for observing the reading on the scale. The correct reading is obtained when the eye is kept vertically above the scale mark to be read. This avoids what is known as the parallax error. Thin scales introduce smaller parallax errors, therefore, metre scales are usually made very thin or have bevelled edges.
After noting the reading corresponding to one end of the object, shift the eye to the other end of the object and note the reading corresponding to the second end. Obviously, the difference of these two readings would give us the length of the object.
While using a metre scale, the following precautions should be observed:
- Since the edges of the scales are likely to be in a worn-out state, it is better to follow the difference method to measure the length of a given object.
- Keep the line of sight vertical to avoid the error due to parallax.
A metre scale is usually graduated in millimetres. Hence, it cannot be used to measure lengths smaller than and better than 1 mm. We say that the least count of the meters scale is 1 mm. Thus,
The least count of a measuring device is the smallest measure that it can measure.
Every measuring device has some least count, hence it cannot measure better than its least count. For example, suppose one end of the object is at 2.0 cm, while the other end does not coincide with any of the markings of the metre scale, say it is between 6.1 cm and 6.2 cm. Clearly, then the length of the object is somewhat greater than 4.1 cm, but we are not able to measure the amount by which the length is greater than 4.1 cm.
For measuring lengths with greater accuracy, we use an additional appliance known as a vernier. Thus, by using a vernier device, lengths can be measured with a better least count.
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