Theory:

In day-to-day life, we use a metre scale for measuring lengths.
 
pxfuel.com (1).jpg
Metre scale
 
We use this metre scale to measure the length of dress, wood, etc., Measuring scales are calibrated in \(cm\) and \(mm\) scales.
 
Remember that the smallest length, which can be measured by metre scale, is called least count.
 
Generally, the least count of a scale is \(1\) \(mm\). We can measure the length of objects up to millimetres accuracy using this scale.
 
But using this metre tape, can you find the length of the cricket ball?
 
We cannot accurately determine the length of the cricket ball as it is in a spherical shape.
 
The metre scale is not sufficient for measuring the size of small spherical objects; therefore, Vernier caliper and Screw gauge are used.
 
Now we will see how Vernier caliper and screw gauge are used to find the length of objects.
 
Properties
Vernier caliper
Screw gauge
DescriptionVernier.PNGScrew cage.PNG
 
 
S.NoPropertiesVernier caliperScrew gauge
1Used for
Vernier caliper can measure the inner and outer diameters of objects.
Screw gauges are used to measure the length and thickness of very small objects.
2Least countLeast count of Vernier caliper is 0.01 cm 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 possess zero error.
If the zero mark on the circular scale is coinciding baseline 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 a positive error.
But if zero mark on the circular scale is comes below the baseline of the main scale, then the instrument possess  \((+)\) a 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.
In case the zero marks on the circular scale is coming above the base line of the main scale, then the instrument possess  \((-)\) a negative zero error.
Reference:
https://www.pxfuel.com/en/free-photo-oxcpz/download