Answer

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**Hint**It should be known to us that the potentiometer is defined as a three-terminal resistor which has a sliding or rotating contact which forms an adjustable voltage driver. If only 2 terminals are used then in that case one end, and the wiper will act as a variable resistor or a rheostat.

**Step-by step answer**

We know that:

In case of internal resistance measurement by potentiometer the ratio between the two voltages are given as:

$\dfrac{{{V_1}}}{{{V_2}}} = \dfrac{{{l_1}}}{{{l_2}}} = \dfrac{{\left\{ {\dfrac{{E{R_1}}}{{({R_1} + r)}}} \right\}}}{{\left\{ {\dfrac{{E{R_2}}}{{({R_2} + r)}}} \right\}}} = \dfrac{{{R_1}({R_2} + r)}}{{{R_2}({R_1} + r)}}$

We know that in the above expression the values of the variables are given as: ${l_1} = 2m,\;{l_2} = 3m,\;{R_1} = 5\Omega \;and\;{R_2} = 10\Omega$

Now we have to put the values in the above expression to get the following :

$\Rightarrow \dfrac{2}{3} = \dfrac{{5(10 + r)}}{{10(5 + r)}}$

On the further evaluation we get that the value of r is:

$\Rightarrow 20 + 4r = 30 + 3r$

So we get the value of r as :

$\Rightarrow r = 10\Omega$

**Hence, the correct answer is Option B.**

**Note**We should know that the main principle behind the working of the potentiometer is that the potential dropped across a segment of a wire which is of uniform cross- section carrying a constant current is directly proportional to its length.

In simple words we can say that the potentiometer is defined to be a simple device which is used to measure the electrical potentials.

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