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.
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Make the connections as below. Here the dots are the connection points and cross-marks are the key knobs.
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 $E_1$.