Theory:

In day to-day life, we use meter scale for measuring lengths.
 
Metertape.png
 
We use this meter scale to measure the length of dress, wood etc.They are calibrated in cm and mm scales.
 
Remember that the smallest length which can be measured by meter scale is called least count.
 
Generally the least count of a scale is \(1\) mm. We can measure the length of objects up to millimeter accuracy using this scale.
 
But using this meter tape can you find the length of the cricket ball?
 
We cannot accurately determine the length of the cricket ball because that ball is in spherical shape.
 
Therefore the meter scale is not sufficient for measuring the size of small spherical objects. And So, Vernier caliper and screw gauge are used.
 
Now we will see how Vernier caliper and screw gauge, which are used to find the length of objects.
 
PropertiesVernier caliperScrew gauge
DescriptionVernier.PNGScrew cage.PNG
 
 
S.NoPropertiesVernier caliperScrew gauge
1Used for
Vernier caliper which can measure the inner and outer diameters of objects.
 
Screw gauges are used measure the length and thickness of very small objects.
 
 
2Least countLeast count of Vernier caliper is 0.01cm Least count of Screw gauge is 0.01 mm
3Zero errorObserve if the zero marking of the main scale coincides with that of the Vernier scale. If they are not coinciding with each other, the instrument is said to posses zero error.
If the zero mark on the circular scale is coincide base line of the main scale, then the instrument is said to posses no zero error.
4\((+)\) Positive zero error.If the zero mark of the Vernier is shifted to the right, it is called positive error.
But if zero marks on the circular scale is comes below the base line of the main scale, then the instrument posses  \((+)\) Positive zero error.
5\((-)\) Negative zero error.Similarly, if the zero mark of the Vernier zero is shifted to the left of the main scale zero marking, then the error is negative.
Incase the zero marks on the circular scale is comes above the base line of the main scale, then the instrument posses  \((-)\) Positive zero error.