Spherometer Video Lecture - Class 11

spero meter a sparrow meter is a
precision instrument to measure the
radius of curvature of the spherical
surface the sparrow meter consists of a
micrometer screw threaded into a small
tripod with a vertical scale fastened to
it
the head of the screw has a graduated
disc of hundred divisions used to
measure fractional turns of the screw
let's find the principle used in the
sparrow meter the thickness T of the
glass strip can be determined using the
equation T is equal to n into pitch plus
X into least count where n is the number
of complete rotations need by the disc
taking initial reading of circular scale
as reference point which is the distance
moved by the number of rotations X is
the number of additional circular scale
divisions as an excess of complete
rotations LC is the least count of the
sparrow meter the radius of curvature of
the given spherical surface using the
relation R is equal to L square divided
by 6 h plus h divided by 2 where L is
the distance between any of the two legs
H is the perpendicular distance between
the legs and screw of the sparrow meter
when it is above the spherical surface
materials required spirometer spherical
surface plane glass slab glass strip a
sheet of paper a ruler procedure arrange
the apparatus on the table to find the
least count of the spirometer the pitch
of the screw needs to be calculated
initially using the relation P is equal
to the distance moved divided by the
number of rotations if there are n
divisions in the circular scale then the
least count of the screw LC is equal to
P divided by n which is equal to 1 by
100 that equals point not 1 millimeter
to find the distance between the legs of
the sparrow meter get the pricks of the
three legs a b and c on a white piece of
paper the mean distance of a B BC and AC
will give the distance between the legs
L raise the screw sufficiently upwards
and place the speedometer on the
spherical surface which is placed on a
plain glass slab so that it's three legs
rest on it gently turn the screw
downwards till the screw tip just
touches the spherical surface use a
paper strip for the confirmation note
the reading of the circular disc which
is in line with the vertical scale let's
take a as a reference variable remove
the spherical surface and place the
sparrow meter directly over the glass
slab turn the screw downwards and count
the number of complete rotations made by
the disc till the tip of the screw just
touches the plane surface of the glass
slab use a paper strip for the
confirmation
note the reading of the circular scale
which is finally in line with the
vertical scale let it be B find the
number of circular disk divisions in the
last incomplete rotation now find the
total reading H using the relation H is
equal to n into pitch plus X into least
count calculate the radius of curvature
of the given spherical surface using the
relation r is equal to l squared divided
by 6 h plus h divided by 2
similarly the thickness of the glass
plate can be measured by placing the
glass plate below the screw of the
speedometer while the legs should rest
on the glass slab calculate the
thickness using the equation t is equal
to n into pitch plus X into least count
applications the spirometer is primarily
used by opticians to measure the
curvature of the surface of a lens for
the calculation of its refractive index
it can also be used to measure the depth
of an etched design below the protected
surface of a flat metal plate