# How do I connect a four way key in a potentiometer?

How do I connect (which wire to which terminal) a four way key to a Potentiometer to compare the EMF of two cells. Circuit diagram and a four wayway key is shown in the pictures.

When you want to measure the deflections on the galvanometer produced by $$E_1$$, remove the middle knob of the path along $$E_2$$. By removing a key knob, you are leaving it in an open circuit, and hence no current passes through. If you want to measure the deflections due to $$E_2$$, then remove the middle knob in $$E_1$$.