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Q & A: Measuring devices

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Most recent answer: 10/22/2007
Q:
give me a brief note on vernier calliper, micrometer screwgauge and meter rod and how to calculate there readings?
- arsalan khalil (age 13)
rawalpindi, pakistan
A:
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. . 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 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)

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