Apparatus:
Micrometer screw gauge, Wire or sphere.
Diagram:
Point to Ponder
The screw gauge is an instrument used to measure the thickness of very thin metal strip or the diameter of ball bearing.
It is based upon the principle of a screw and consists of a U-shaped frame having a fixed end a cylinder with moveable end B having uniform grooves passes through the other end. A scale is marked on the cylinder in millimeters is called linear scale. A cap is fitted on ne cylinder and carries on its inner edge a circular scale having 50-100 equal small divisions around it which is called circular scale. This scale is used to measure the fraction of a revolution. The number of complete revolutions can be read on the linear scale which is graduated on the nut parallel to the axis of tle screw.
The reading of the circular scale is taken against a line known as the reference line.
It can measure upto 0.01mm therefore also called micrometer. The linear scale usually graduated in millimeters or half millimeters.
Pitch
The distance advanced by the screw in one complete rotation is called pitch.
Zero Error
The stud and tip of a screw gauge are made to touch each other. If zero link: of circular scale coincides with index line, then zero error will be zero. There are two types of errors:
Positive Error
If zero line of circular scale lies below index line, the error will be positive. In order to find its value, note the no. of circular division coinciding with index line and then multiply it by least count
e.g. No. of circular div. coinciding with index line = 3
Zero error = + 3L .C
= + 3 * 0.01mm
= + 0.03mm
Zero correction = – 0.03 mm
Negative Error
If zero line of circular scale lies above the index line, the error will be negative. In order to find its value, note the no. of circular div. above the index line and multiply it by L.C.
e.g. No. of circular div. above index line = 3
Zero error = – 3 L.C.
= – 3 0.01 = – 0.03mm
Zero correction = + 0.03 mm
Procedure
Find pitch of screw and then least count of screw gauge. Find zero error if possible.
Place the wire between stud A and B and turn screw till wire is gently pressed. Note the reading of last visible division of linear scale, which is denoted by R. Now note the no. of circular scale division coinciding with index line say ‘n’. Multiply ‘n’ by least count, this will five fraction to be added say ‘y’. Add y in R, this will give observed diameter of wire. To find corrected diameter apply Zero correction. Take three more readings at different points of wire. Then find radius of wire and calculate area by using formula.
Α = πr2 l
Similarly find diameter of sphere and hence its radius. Volume of a sphere can be calculated by using formula.
V = 4/3 π r3
Observations and calculations
= 0.5 mm/ 50
= 0.01mm
Diameter of wire
| No.of Obs | Linear scale reading R |
Circular scale reading n |
Fraction to be Added Y = n L.C. |
Observed diameter = R + y |
Corrected Length Z.C |
| mm | div. | mm | mm | mm | |
|
1 2 3 4 |
Mean diameter = d = ………… mm
Radius of wire = r = d/2 = …….. mm
Area of cross-section of wire = A = π r3 = ……. mm2
Result:
Area of cross-section of wire =A = ……. mm2
| No.of Obs | Linear scale reading R |
Circular scale reading n |
Fraction to be Added Y = n L.C. |
Observed diameter = R + y |
Corrected Length Z.C |
| mm | div. | mm | mm | mm | |
|
1 2 3 4 |
Mean diameter = d = ………… mm
Radius of sphere = r = d / 2 = …….. mm
Volume of sphere = V = 4 / 3 r3 = ……. mm3
Result:
Volume of sphere = V = ……. mm3
Precautions:
(1) Apply the zero error in the reading.
(2) Do not press the screw too much.
(3) The screw should always be turned in the same direction.
VIVA VOCE
Q.1 What do you mean by pitch of screw gauge?
Ans. When circular scale completes one rotation, the distance covered on linear scale is called pitch
Q.2. What is the screw gauge?
Ans. It is the instrument used to measure the diameter of an object with accuracy.
Q.3 Calculate least count of screw gauge if its circular scale has 100 division.
Ans. Least count = Pitch of screw / No.of circular div.
= 1 mm/ 100 = 0.01 mm
So, least count of screw gauge = 0.01 mm
