Measuring Instruments:
Measuring
instruments are used to measure various physical quantities such as length,
mass, time, volume etc.
·
Meter Rule:
A meter rule is a length measuring instrument. It is commonly used in the
laboratories. It is one meter long which is equal to 100 cm. Each cm
Is divided into 10 small divisions called millimeter. Thus one millimeter
is the smallest reading that can be taken by using a meter rule and is called
its least count.
·
Measuring Tape:
Measuring tapes are used to
measure length in meter and centimeters. A measuring tape consists of a thin
and long strip of cotton, metal or plastic generally 10m, 20m, 50m, or 100m
long.
·
Vernier Calipers:
The accuracy obtained in
measurements using a meter rule is upto 1 mm. However an accuracy greater than
1 mm can be obtained by using some other instruments such as a vernier
calipers. A vernier calipers consists of two jaws. One is a fixed jaw with main
scale fixed on it. The other jaw is a moveable jaw. It has vernier scale having
10 divisions on it. These are in such a way that each of its division is 0.9
mm. The difference between one small division on main scale division and one
vernier scale division is 0.1 mm.
L.C = 0.1
mm
·
Zero Error:
Zero
error will exist if zero line of the vernier scale is not conceding with the
zero of main scale.
·
Zero Correction:
Knowing
the zero error, necessary correction can be made to find the correct
measurement. Such a correction is called zero correction of the instrument.
·
Positive Zero Error:
Zero
error will be positive if zero line of vernier scale is on the right side of
the zero of the main scale.
·
Negative Zero Error:
Zero
error will be negative if zero line of vernier scale is on the left side of the
zero of the main scale.
Screw Gauge:
A
screw gauge is an instrument that is used to measure small lengths with
accuracy greater than vernier calipers. It is also called as micrometer screw
gauge. A simple screw gauge consists of these parts.
·
U-Shaped metal frame:
U-shaped metal frame with a metal stud at its one end.
·
Hollow Cylinder:
A hollow cylinder has a millimeter scale over it along a line called
index line parallel to its axis. The hollow cylinder acts as a nut.
·
Thimble:
A thimble has a threaded spindle inside it. The distance between
consecutive threads on the spindle is 1 mm. This distance is called the pitch
of screw gauge on the spindle.
Least Count = 
L.C = 
L.C = 0.01mm =
0.001cm
Zero
Error:
To find the zero error, close the gap between the
spindle and the stud of the screw gauge by rotating the ratchet in the
clockwise direction. If zero of circular scale coincides with the index line,
then the zero error will be zero.
Positive
zero error:
Zero error will be positive if zero
of circular scale is behind the index line
Negative
Zero error:
Zero error will be negative if zero of circular scale has
crossed the index line.
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