A Wheatstone bridge is used to determine the value of unknown resistance $X$ by adjusting the variable resistance $Y$. For the most precise measurement of $X$, the resistances $P$ and $Q$:

For a Wheatstone bridge to be most sensitive and precise, the resistances in all four arms should be of the same order of magnitude. If $P\approx Q$, the bridge is balanced most accurately near the midpoint of the slide wire (if applicable) or provides the best detection of null point. Small and equal resistances minimize errors due to temperature variations and lead resistances.