Remember
that while looking at the reading on a scale, we must keep
our eyes in front of and in line with the reading to be
taken. In case of a metre scale, it is not always possible
to make the zero mark on the scale coincident with one end
of the line to be measured. With repeated use, the ends
of measuring scale get somewhat worn-out and ill defined.
In such cases, we keep the metre-scale with any of its divisions
other than zero coincident with one end of the line. Suppose
we place the scale (ruler) in such a way that the two ends
of the line segment coincides with 2.0 cm and 4.2 cm marks,
respectively (Fig 1.4). Then, the length of the line segment
is
4.2 cm – 2.0 cm = 2.2 cm
That is, it is the difference between the readings on a
scale at its two ends.
The
length of a line segment is the difference between the readings
on a scale at its two ends
If we have to measure a larger length, such as length of
a playground, we use a measuring tape that may be 10 m,
15 m or 50 m long.
Sometimes, we are required to measure very small lengths,
say (less than 1 mm) like the diameter of a thin wire, the
dimensions of fine machine parts, etc. We cannot use a metre
scale for such measurements. For such distances, measuring
instruments like the vernier callipers and the screw gauge
are used. Vernier callipers, as shown in Fig 1.5, is an
instrument used to measure the length or thickness of a
solid body up to 0.01 cm accurately. A screw gauge as shown
is Fig. 1.6, is an instrument used to measure the length
or thickness of a solid body up to 0.001 cm. accurately
However, each measuring instrument is limited to a certain
accuracy of measurement which depends on its graduation.
To measure the thicknessof a wire or a metallic sheet we
require screw gauge.
Vernier
calliper
Screw
gauge
To
measure large distances like the distance of your school
from your house, or distance between two cities or the distance
between the earth and moon, we use indirect methods of measurement.
For example, to measure the distance between two cities,
we will measure the average speed of certain vehicle, say
a car, and the time taken by it to cover that distance.
The product of the speed and time will give the required
distance.
