consists
of a u-shaped still frame F (1.3).
Fig
1.3 Screw Gauge
The
rod with plane end is permanently fixed with the plane surface A of one arm.
The
other
arm has a hollow cylinder C. A linear scale graduated in millimeter is marked
on
the
cylinder and a screw with a cylindrical cap T is fitted with it. The screw can
moved
through
the hollow cylinder C. The end of the cylindrical cap T is graduated 0 to 50 or
100.
When the Jaws are closed, that is, then the head of the screw B touches the
plane
end
of the fixed rod A, the zero of the circular scale coincides with the zero of
the linear
scale.
If the zero marks of the two scales do not coincide, then there is instrumental
error.
The
distance through which the screw advances along the linear scale when the cap T
is
rotated
once is called the Pitch of the screw gauge. The distance through which the
screw
advances when the circular scale is rotated through only 1 division is called
the
least
count. The least count is obtained by dividing the pitch by the number of
divisions
in
the circular scale.
Example : Suppose, the end B of the rod has crossed 12mm mark of the
main scale and 7th
vernier
mark has coincide with a main scale mark. Then the length of the rod will be
L
= 12mm + 7 x 0.1mm (here vernier constant is 0.1mm)
=
12.7mm = 1.27cm.
Physics
17
Therefore,
Least count =
Pitch
no.
of division of the circular scale.
Usually
the circular scale has 100 divisions and the pitch is 1mm.
Least
count =
1
100mm
= 0.01mm.
Measurement by Screw Gauge :
The
wire whose diameter is to be measured or the plate whose thickness is to be
determined
is placed between A and B. The wire or the plate should be so placed that it’s
one
side touches A and the other side touches B. Now the reading of the linear and
circular
scales is to be taken. Suppose the reading of the linear scale 1mm, and the
number
of divisions of the circular scale is C. Then the diameter of the wire or the
thickness
of the plate will be :
Diameter
or thickness = Linear scale reading + no. of divisions of the circular scale x
Least
count.
That
is, D = L + C x LC
When
the head of the screw touches the plane end of the fixed rod A, then the zero
of the
circular
scale should coincide with the zero of the linear scale, then it means there is
an
error. For
this reading should be corrected.
