Theory
In screw gauge, the rotational motion is proportional to the linear motion i.e., for each complete rotation of the circular scale (clockwise or anti clockwise), the screw head moves equal linear distances.
Screw pitch is the linear distance moved by the screw head for one complete rotation.
Least count of the screw gauge is given by
least \hspace{0.1cm} count=\frac{screw \hspace{0.1cm} pitch}{number \hspace{0.1cm} of \hspace{0.1cm} divisions \hspace{0.1cm} of \hspace{0.1cm} circular \hspace{0.1cm} scale}
The total reading of screw gauge = main scale reading + ( circular scale reading \times least
If d be the diameter of a given wire, then area of cross section is
A=\frac{\pi d^{2}}{4}
Apparatus
i. Screw gauge, ii. Given wire
Results
1 division of linear scale= ____ mm
Screw pitch = ______ mm
Number of division of circular scale =
least \hspace{0.1cm} count=\frac{screw \hspace{0.1cm} pitch}{number \hspace{0.1cm} of \hspace{0.1cm} divisions \hspace{0.1cm} of \hspace{0.1cm} circular \hspace{0.1cm} scale}
Zero error = ______ mm
Table for determination of diameter and area of cross section
| No. of Obs | Linear scale reading (mm) | Circular scale reading | Total reading (mm) | Mean diameter (d1) (mm) | Corrected diameter (d=d1-e) | Area of cross section (cm2 ) |
| 1 2 3 4 5 6 7 8 | ____ mm =_____ cm |
Precautions and Discussions
1. The should not be pressed very tightly.
2. Zero error should be corrected properly.
3. The diameter should be measured at different places as the cross sectional area may not be uniform in all places of the given wire.
Viva Questions on screw gauge practical
A screw gauge is an instrument used for the measurement of diameter of thin wire or the thickness of metal sheet.
The pitch of a micrometer screw is the linear distance moved by the screw head for one complete rotation.
least \hspace{0.1cm} count=\frac{screw \hspace{0.1cm} pitch}{number \hspace{0.1cm} of \hspace{0.1cm} divisions \hspace{0.1cm} of \hspace{0.1cm} circular \hspace{0.1cm} scale}
If the zero of the circular scale does not coincide with the zero of the linear scale when the the screw is fully tighten, then the error is called zero error.
This instrument is also called micrometer because it can measure the dimension of an object of the order of micro meter.
It is used to measure the diameter of a thin wire accurately.
Micrometer screw gauge works on the principle of the screw. As the screw rotates, the main scale moves in linear direction.
See the procedure.
If we rotate the screw in reverse direction, the tip of the screw does not move immediately. This error is called backlash error.