1. The potential difference between the ends of the potentiometer wire or the emf of the cell connected in the main circuit may not be greater than the emf of the cells whose emf are to be compared.
2. The positive terminals of the cells and the battery used in the circuit might not be connected to the same end of the potentiometer wire.
3. The rheostat might not be adjusted properly and kept in such a position that it offers minimum resistance to the circuit and allows maximum current through the wire.
