electricity - Potential drop in a wire having negligible resistance

What causes a potential drop in a resistor or load? Why a wire having neglible resistance have same potential across it? The given figure is of a stretched wire potentiometer. My question is why point a and A have same potential after connection through a wire of negligible resistance. Before connection there can be different potentials on poinrs a and A. So how the same potential achieved after connection. I wants it's machanism and explanation.

Answer

The potential difference, or voltage, between two points equals the work required per unit charge to move the charge between the two points. If there is negligible resistance between points a and A, the work required to move charge between the two points (potential difference) is also negligible.

Hope this helps.