Aim: To measure the thickness of the given plate with the help of a spherometer.
Apparatus required: Spherometer, a base plate (e.g. a glass sheet) and given
plate.
Theory:
The spherometer is placed on a plane base plate and the experimental plate
is inserted between the tip of the screw and the glass plate. Then the circular scale
is rotated till the screw tip just touches the surface of the experimental plate. The
circular scale division in just line with the vertical linear scale is noted. Then the
experimental plate is removed and the screw is moved downward till its tip just
touches the base plate. In doing so, the number of complete rotations made by the
circular scale is noted. Also the additional circular scale division moved by the
circular scale in excess of complete rotations is noted.
If ‘n’ is the number of complete rotations and ‘m’ be the additional circular
scale divisions, then the thickness ‘t’ of the experimental plate is given by
t = (n  pitch + m  L.C.)
Procedure:
1. Determine the pitch and the total number of circular scale divisions then
calculate the least count.
2. Place the spherometer on the base plate such that its three legs rest on it
comfortably.
3. Now move the screw upward sufficiently so that the experimental plate
could be placed below the screw tip.
4. Now rotate the circular scale downward till the screw tip just touches the
experimental plate. To test it, try to insert a thin paper piece below the tips of
the screw and the legs. If the paper cannot be inserted, then it ensures that
the tip just touches the surface of the experimental plate.
5. Note the circular scale division which is just in line with the vertical linear
scale. This gives the initial circular scale division.
6. Now remove the experimental plate gently. Turn the screw downward and
at the same time count the number of complete rotations made by the
circular scale, taking initial circular scale division as the reference point, till
the screw tip just touches the base plate.
7. Note the final circular scale division which is just in line with the vertical
linear scale.
8. Determine the number of additional circular scale division moved by
circular scale in excess of the complete rotations.
9. Repeat steps 3 to 8 for five different positions on the same side of
experimental plate extending over the whole plate.
10. Then turn over the experimental plate and repeat the above operations
for its other surface.
Observations:
1.
(a). Pitch of the spherometer, P
= ........ mm
(b). No. of circular scale divisions = ......... mm
(c). Least count
= ......... mm
2. Determination of thickness (t) of the given plate:
No.
of
Obs.
No. of
complete
rotations,
n
Circular scale
Division (C.S.D.)
Initial
Final
(I)
(F)
Additional
C.S.D.
m = (IF )
or
m = (100+I)F
.....
.....
.....
.....
.....
.....
.....
.....
.....
.....
t1
t2
t3
t4
t5
.....
.....
.....
.....
.....
.....
.....
.....
.....
.....
t6
t7
t8
t9
t10
1.
.....
.....
2.
.....
.....
3.
.....
.....
4.
.....
.....
5.
.....
.....
Experimental plate turned over
1.
.....
.....
2.
.....
.....
3.
.....
.....
4.
.....
.....
5.
.....
.....
Thickness
t= nP+(m L.C.)
(mm)
Mean
Thickness
t
(mm)
.....
Calculations:
(Show the calculations for all the thicknesses from t1 to t10. and for the
mean thickness t.)
Result:
The thickness of the given plate = ........ mm = ......... cm
Precautions:
1. The screw should move freely move without any friction.
2. To avoid back-lash error the screw should be moved in the same direction.
3. Readings should be taken at five different places extending over the whole
plate for each surface.
4. While taking the readings always ensure that the tips of the screw and the
legs just touches the experimental plate and base plate respectively.