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Length measurement
Beginner
Rulers, vernier callipers and screw gauges measure lengths at different precision.
Different instruments suit different precisions: a ruler reads to about 1 mm, a vernier calliper to 0.1 mm or 0.05 mm, and a screw gauge to 0.01 mm. For large distances physicists use indirect methods such as parallax or timing a reflected signal.
To find the thickness of one sheet of paper, measure a stack of 200 sheets and divide. Repeating a measurement over many units reduces relative error.
Reading a scale at an angle gives parallax error. Always view the pointer directly from above.
In more detail
Length can be measured with a metre rule, a vernier calliper or a micrometer screw gauge. Each instrument has a different precision, and the smallest division of the scale sets the limit on what it can resolve.
Real-world uses
- Mechanics workshops use micrometers to check that machined parts fit within tolerance.
- Surveyors use tape measures and laser instruments for building plots and roads.
- A carpenter uses a steel rule for quick cuts, while a machinist uses a micrometer for parts that must fit to a few hundredths of a millimetre.
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Frequently asked questions
How do I choose the right ruler?
Pick an instrument whose smallest division is small compared with the size of the change you need to see.
Why does a measuring tape stretch?
Cheap or worn tapes can stretch or warp, which adds a systematic error. Calibrate against a known standard if accuracy matters.
Which length instrument should I use for a classroom experiment?
Use a ruler for lengths of a few centimetres or more, and a vernier calliper or micrometer when you need a precision of a fraction of a millimetre.
Related topics in Measurement & Math Tools
Derived units · Prefixes (milli, kilo, mega) · Scientific notation · Vernier calliper · Screw gauge · Measuring time