關於AL physics的misconception

The basic principle of using a potentiometer to measure the emf of a cell is that at balance, there is no current flowing from the cell. In that case, the emf equals to the terminal voltage of the cell because there is now no voltage drop across the internal resistance.
Just imagine, on the contrary, that if during "balance", there is a current flowing from the cell (which its emf is being measured), this current will pass through the potentiometer wire. The current in the wire thus increases and this alters the calibration of the wire. That is to say, the potential drop per cm of wire will be changed.

If such were to happen, one would adjust the position of the contact point on the wire in order to reduce the current. This adjustment of contact point will continue until it is observed that there is no current flowing from the cell. This is then the balance point.

It is for this reason that in a good potentiometer. The driving cell is generally of higher emf than the cells under measurement. Under such situation, the current flowing through the potentiometer wire is dominately given by the driving cell.