{{Short description, Smallest value a measuring instrument can measure Scale least count

In the science of measurement, the least count of a measuring instrument is the smallest value in the measured quantity that can be resolved on the instrument's scale. William Woolsey Johnson ''The Theory of Errors and Method of Least Squares'', Press of I. Friedenwald, 1890; page 1 The least count is related to the precision of an instrument; an instrument that can measure smaller changes in a value relative to another instrument, has a smaller "least count" value and so is more precise. Any measurement made by the instrument can be considered repeatable to no less than the resolution of the least count. The least count of an instrument is inversely proportional to the precision of the instrument. For example, a

sundial A sundial is a horological device that tells the time of day (referred to as civil time in modern usage) when direct sunlight shines by the apparent position of the Sun in the sky. In the narrowest sense of the word, it consists of a f ...

may only have scale marks representing the hours of daylight; it would have a least count of one hour. A

stopwatch A stopwatch is a timepiece designed to measure the amount of time that elapses between its activation and deactivation. A large digital version of a stopwatch designed for viewing at a distance, as in a sports stadium, is called a stop clock. ...

used to time a race might resolve down to a hundredth of a second, its least count. The stopwatch is more precise at measuring time intervals than the sundial because it has more "counts" (scale intervals) in each hour of elapsed time. Least count of an instrument is one of the very important tools in order to get accurate readings of instruments like vernier caliper and screw gauge used in various experiments. Least count uncertainty is one of the sources of

experimental error Observational error (or measurement error) is the difference between a measured value of a quantity and its true value.Dodge, Y. (2003) ''The Oxford Dictionary of Statistical Terms'', OUP. In statistics, an error is not necessarily a " mista ...

in measurements. Least count of a

vernier caliper A caliper ( British spelling also calliper, or in plurale tantum sense a pair of calipers) is a device used to measure the dimensions of an object. Many types of calipers permit reading out a measurement on a ruled scale, a dial, or a digital d ...

is .02 mm and least count of a

micrometer Micrometer can mean: * Micrometer (device), used for accurate measurements by means of a calibrated screw * American spelling of micrometre The micrometre ( international spelling as used by the International Bureau of Weights and Measures; ...

is .01 mm.

Least count error

The smallest value that can be measured by the measuring instrument is called its least count. Measured values are good only up to this value. The ''least count error'' is the error associated with the resolution of the instrument. A metre ruler may have graduations at 1 mm division scale spacing or interval. A

Vernier scale A vernier scale, named after Pierre Vernier, is a visual aid to take an accurate measurement reading between two graduation markings on a linear scale by using mechanical interpolation, thereby increasing resolution and reducing measurement unce ...

on a caliper may have a least count of 0.1 mm while a micrometer may have a least count of 0.01 mm or 10 microns. The least count error occurs with both systematic and random errors. Instruments of higher precision can reduce the least count error. By repeating the observations and taking the arithmetic mean of the result, the mean value would be very close to the true value of the measured quantity.

References

Measurement