Two cells of electromotive forces (e.m.f.s.) approximately 5 V and 10 V are to be accurately…

Two cells of electromotive forces (e.m.f.s.) approximately 5 V and 10 V are to be accurately compared using a potentiometer of length 400 cm. The student sets up the circuit as shown in the diagram. However, he is not able to measure the ratio of e.m.f.s correctly. Analyse the circuit shown and spot a minimum of two errors in the circuit. Explain with proper reasoning.