Can you show how vernier callipers and screw gauge works ?
Dear Student,
Vernier Calliper
For measuring length accurately, Vernier calliper is used. Vernier calliper was invented by Pierre Vernier. It has accuracy up to 0.01 cm.
Essential Parts:
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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.
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Vernier Scale
It is a small movable scale, which slides along the main scale. It has 10 divisions adding up to 9 mm length.
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External jaws
J1 and J2 are called external jaws because they are used for measuring external dimensions.
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Internal jaws
J3 and J4 are collectively called internal jaws and are used for measuring internal dimensions of a hollow object.
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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 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.
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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.
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Negative Zero Error
If the zero mark of the Vernier scale lies on left hand side of zero mark of the main scale, then the error is said to be negative and correction is said to be positive.
| Correction = (n − Coinciding division of V.S.) × L.C. |
Where, n is the total number of Vernier scale divisions
Screw gauge
A micrometer screw gauge is used for measuring much smaller lengths than what a Vernier calliper can measure. It works with an accuracy of 0.001 cm. It is mainly used for measuring the diameter of thin wires.
Construction
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U-frame
It is a U-shaped steel frame with stud (fixed jaw) at one end and a nut at the other end.
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Nut and screw
The nut is threaded from inside and the screw from outside.
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Thimble and circular cylinder
The screw is connected to a hollow circular cylinder (S), which rotates along with nut on turning.
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Sleeve cylinder
It is a hollow cylinder attached to the nut.
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Base line
Commonly called main scale or sleeve scale, it is the graduated line parallel to the axis of the nut.
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Circular scale or thimble scale
It is the hollow half cone moving over the sleeve cylinder.
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Ratchet
It is attached to screw by means of a spring.
Terms and Definitions:
Pitch of Screw: It is the distance between two consecutive threads of the screw, measured along the axis of the screw.
Least count of Screw: It is the smallest distance moved by its tip when the screw turns through 1 division marked on it.
Formula for measuring a diameter
The diameter of a wire is measured by using the formula,
| Observed diameter = Main scale reading + Circular scale reading × L.C. |
Of course the zero error, if any, has to be added or subtracted accordingly.
Zero Error:
When stud A and screw end B are joined and zero of main scale does not coincide with that of the circular scale, the screw gauge is said to have a zero error.
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Positive zero error
When zero line on circular scale is below the reference line, there is a positive zero error and correction is negative.
| Correction = −Coinciding division of C.S. × L.C. |
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Negative zero error
When zero line on circular scale is above the reference line, there is a negative zero error and correction is positive.
| Correction = (n − Coinciding division of C.S.) × L.C. |
Where, n is the total number of circular scale divisions.
Regards