Measurement and Measuring Instruments
Measurement of Length- Lab Method
For measuring length of a rod, diameter of a sphere, depth of small bottle, etc., vernier calliper is used. Vernier calliper was invented by Pierre Vernier. It has an accuracy up to 0.01 cm.
Principle: The vernier calliper consists of two scales: main scale with jaw J1 and vernier scale with jaw J2. The main scale is graduated with division of 1 mm each. The vernier scale graduation is such that the length of n division on it is equal to the (n-1) divisions of the main scale. The vernier scale can slide over the fixed main scale such tha the vernier scale can be fixed at any position on the main scale with the help of a screw S.
It is a thin long strip of steel on which a scale in engraved in mm. The main scale is generally 15 cm long.
It is a small movable scale, which slides along the main scale. It has 10 divisions adding up to 9 mm length.
J1 and J2 are called external jaws because they are used for measuring external dimensions.
J3 and J4 are collectively called internal jaws and are used for measuring internal dimensions of a hollow object.
It is at the end of main scale and slides along with the Vernier scale.
When jaws J1 and J2 are closed, the zero mark of the main scale coincides with that of the Vernier scale and 19th main scale divisions coincide with 20th Vernier scale divisions.
Terms and Definitions:
The smallest value of length, which can be acquired directly from the main scale accurately, is called pitch.
The magnitude of the smallest measurement, which can be measured by an instrument accurately, is called its least count.
Formula for measuring a length
Length of an object is found by using the formula,
Length = (Pitch × Main Scale Division) + (Least Count × Vernier Scale Division)
Zero Error of Vernier Calliper
If the zeroes of M.S. and V.S. do not coincide, then the instrument is said to have a zero error.
Positive Zero Error
If the zero mark of the Vernier scale lies on right hand side of zero mark of the main scale, then the error is said to be positive and correction is said to be negative.
Correction = −Coinciding division of V.S. × L.C.
This correction has to be added to the measured length.
Negative Zero Error
If the zero mark of the Vernier scale lies on left hand side of zero mark of the main scale, then t…
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