To find the value of an unknown resistance by using a meter bridge,why are the positions of $R$ and $S$ interchanged?

(N/A) In a meter bridge experiment,the positions of the known resistance $(R)$ and the unknown resistance $(S)$ are interchanged to eliminate the end errors.
End errors occur due to the resistance of the copper strips at the ends of the meter bridge wire and the contact resistance at the terminals.
By interchanging the resistances,we take two sets of readings.
Let the true value of the unknown resistance be $S$ and the end errors be $\alpha$ and $\beta$.
In the first case,the balance condition is $\frac{R}{S+\alpha} = \frac{l_1}{100-l_1}$.
In the second case,after interchanging,the condition becomes $\frac{S}{R+\beta} = \frac{l_2}{100-l_2}$.
By taking the average of the two measurements,the systematic errors caused by the end resistance are effectively cancelled out,leading to a more accurate result.