Question

Find the least count of a milliammeter in which there are $20$ divisions between $400mA$ and $500mA$.

Answer 1

In the science of measurement, the least count of a measuring instrument is the smallest and accurate value in the measured quantity that can be resolved on the instrument's scale. The formula of least count is,
$\text{Least count} = \dfrac{\text{Range}}{\text{Number of divisions}}$
So, we are to determine the range and no. of divisions of the milliammeter and use the above formula to find the least count of the milliammeter.

Complete step by step answer:
Given, there are $20$ divisions in the milliammeter between $400mA$ and $500mA$.
Therefore, the no. of divisions of the milliammeter $= 20$
And, the range of the milliammeter $= \left( {500 - 400} \right)mA$
$= 100mA$
Therefore, by using the formula,
$\text{Least count} = \dfrac{\text{Range}}{\text{Number of divisions}}$
We can determine, the least count, as
$\text{Least count} = \dfrac{100mA}{20}$
$\Rightarrow \text{Least count} = 5mA$
Therefore, the least count of a milliammeter in which there are $20$ divisions between $400mA$ and $500mA$ is $5mA$.

Note:
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. Least count of an instrument is one of the very important tools in order to get accurate readings of instruments like vernier calliper and screw gauge used in various experiments. Least count uncertainty is one of the sources of experimental error in measurements. And, an ammeter (from ampere meter) is a measuring instrument used to measure the current in a circuit. Electric currents are measured in amperes (A), hence the name. Milliammeter is an instrument that measures the electric current in a circuit in milliamperes.