Hi Arsalan,
Nice questions about measuring devices. Experimental physicists
are constantly concerned about measuring quantities in nature,
understanding the accuracy of the measurements, and improving the
accuracy. A vernier caliper is a device for measuring the size of
objects more accurately than one could just by using a meter stick or
other kind of ruler. It usually contains two sets of jaws -- one which
fit on the outsides of objects, and another for measuring the insides
(a picture on the web site below should clear up why you need two
separate jaws for this). The vernier part increases the accuracy of the
measurement by introducing a second scale which slides along the main
scale. The second scale has division markings with a spacing only a
little bit different than the main division's scale, which means that
in general they won't line up, except at one point. A tiny motion of
the caliper's jaws will make a different pair of marks on the two
scales line up. To get a feel for it, you need to see one in action and
read a longer description.
Here's one on the web.
The accuracy of a measurement using a vernier caliper is about 0.1 mm,
which is somewhere close to the visual acuity of someone with good
eyes.
A micrometer screw gauge looks very much like a C clamp, but the
threads are very closely spaced. There is a scale marked on the handle
that you turn to adjust the gap. A coarse scale is on the rod, and the
fine scale is on the rotating part. A picture clears it all up. Here is
a
web site
with pictures and instructions for both vernier calipers and micrometer
screw gauges. One thing to worry about with the screw gauge more than
the caliper is that it is very easy to apply pressure to whatever it is
your're measuring and crush it a bit, distorting the measurement. Screw
gauges are made to limit this, but if the measured object is soft or
crushes easily, it is important to watch out for this.
A meter rod sounds like a rod with markings on it (usually with
fine markings every millimeter, and bigger markings every centimeter,
and sometimes intermediately big ones on the 5 mm marks). It is the
least accurate of the three devices for making distance measurements,
but can be used to measure objects much bigger than you can get in the
jaws of a vernier caliper. One way around this is just to make bigger
calipers, but then the uncertainty due to thermal expansion of the
scales would be bigger than the uncertainty of reading the vernier
scale, and people could be lulled into thinking that their measurements
are more accurate than they really are.
Tom
(published on 10/22/2007)
