Each instrument has an inherent amount of uncertainty in its measurement.
Even the most precise measuring device cannot give the actual value
because to do so would require an infinitely precise instrument.
A measure of the precision of an instrument is given by its uncertainty.
As a good rule of thumb, the uncertainty of a measuring device is 20% of the
least count. Recall that the least count is the smallest subdivision given
on the measuring device. The uncertainty of the measurement should be
given with the actual measurement, for example, 41.64 ± 0.02cm.
Here are some typical uncertainties of various laboratory instruments:
- Meter stick: ± 0.02cm
- Vernier caliper: ± 0.01cm
- Triple-beam balance: ± 0.02g
- Graduated cylinder: 20% of the least count
Here's an example. The uncertainty of all measurements
made with a meter stick whose smallest division (or least count) is one
millimeter is 20% of 1mm or 0.02cm. Say you use that meter stick to
measure a metal rod and find that the rod is between 10.2 cm
and 10.3cm. You may think that the rod is
closer to 10.2cm than it is to 10.3cm, so you make your best
guess that the rod is 10.23cm in length. Since the uncertainty in
the measurement is
0.02cm, you would report the
length of the metal rod to be 10.23 ± 0.02cm (0.1023 ± 0.0002 m).
When a quantity is graphed, it is common for the uncertainty of that
quantity to be represented by error bars. For more information about
error bars, see our Excel tutorial
on using error bars.