9th Class Physics Notes “Screw Gauge”
Screw gauge:
Linear scale/main scale:
- Linear or main scale of screw gauge is graduated up to 25mm, this scale have a line called datum line, on which the millimeter scale is graduated.
Circular scale:
- The scale which is in form circle over the main scale is called circular scale.
- The circular scale is divided into 50 or 100 equal divisions.
Anvil:
- The u-shaped frame have fix study with a plane round end is known as anvil.
Spindle:
- At the right side of u-shaped frame there is a more able spindle with its plane round end.
Ratchet:
- A screw gauge consist of a ratchet, with help of this ratchet the circular scale can be rotated.
Pitch of the screw:
- The distance covered by main scale after giving one complete rotation to the circular scale is called pitch of the screw.
- Pitch of the screw = 0.5mm or 1mm
Least count:
- The minimum distance that can be measure with help of micrometer is called least count.
- Least count of micrometer is 0.01mm.
Determination of least count of micrometer:
The least count of micrometer can be obtained byas follow:
least count = pitch of screw / total division on circular scale
If the number of division on the circular scale is ’50’, then the pitch of screw is 0.5mm.
So,
least count = 0.5mm / 50
Zero error:
After closing the screw gauge checks it, if the datum line of the main scale coincides with zero of circular scale, there is no zero error.
If the datum line of the main scale do not coincide with the zero of circular scale there is zero error.
Types of error:
- Positive zero error
- Negative zero error
- Positive zero error:
When the screw gauge is closed and the zero of the circular scale left behind(below) the datum line of the main scale the error is known as positive zero error.
Positive zero error correction:
For correction of positive zero error first we find the error, and subtract it from the final reading.
- Negative zero error:
When the screw gauge is closed and the zero of the circular scale go ahead (above) from the datum line of the main line the zero error is known as negative zero error.
Negative zero error correction:
For correction of negative zero error first we find the error, and then add it with the final reading.
Measurement by screw gauge:
To find the thickness of a wire by using screw gauge, we follow the following method.
- First of all remove the zero error, if the error is in the instrument.
- Placed the object b/w the spindle and anvil.
- Read the main scale reading. For example the edge of circular scale is b/w 5 and 6 millimeter i.e. the thickness of wire is greater than 5mm and less then 6mm i.e.
main scale reading = 5mm
Explanation:
Let we want to measure the length of a box, with help of meter rule, which is graduated in centimeter as well as in millimeter.
Let the length of box is measured by three person, and they recorded its length as, 12.67cm, 12.65cm and 12.68cm.
Here in this measurement the three person are agree with 12.6 cm, so these digit are known as accurately known digit, but about forth digit all the three person are not agree. We are in doubt that which digit is correct in the 7 and 8 digit, such digit is known as doubtful digit.
So in this measurement all the four digits are significant, in which first three are accurate known and forth digit is doubtful.
Rule for determine the significant figure:
The general rule for determine the significant figure are under.
Rule 1:
- All non-zero digit 1,2,……9 are significant figure.
Example:
- For example in 12.48 and 148 there is four and three significant figure.
Rule 2:
- The zero between two non-zero is significant.
Example:
- For example 1204 there are four significant figures.
Rule 3:
- The zero on left side of non-zero is not significant.
Example:
- For example 0.0041 there are two significant figures.
Rule 4:
- In decimal factor zero to the right are significant.
Example:
- For example in 3.570 and 7.400 figure all the digit are significant.
Rule 5:
- In non-decimal factor, zero to the right of a significant digit may or may not be significant depend upon the counting unit of measuring device.
Example:
The integer 6000 kg if the measuring scale has a least count 1kg. then there are 4 significant digits. i.e
6000kg……..1kg……..4 digits
Similarly if the least count is 10 kg then
6000kg……..10kg……..3 digits
If least count is 100 kg then
6000kg……..100kg……..2 digits