Measurement and Measuring Instruments

Measurement of Length- Lab Method

Vernier Calliper

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.

Essential Parts:

1. Main Scale

It is a thin long strip of steel on which a scale in engraved in mm. The main scale is generally 15 cm long.

2. Vernier Scale

It is a small movable scale, which slides along the main scale. It has 10 divisions adding up to 9 mm length.

3. External jaws

J₁ and J₂ are called external jaws because they are used for measuring external dimensions.

4. Internal jaws

J₃ and J₄ are collectively called internal jaws and are used for measuring internal dimensions of a hollow object.

5. Tail

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 19^th main scale divisions coincide with 20^th Vernier scale divisions.

Terms and Definitions:

Pitch The smallest value of length, which can be acquired directly from the main scale accurately, is called pitch. Least Count 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.

1. 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.

2. 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…