[Fatma Adel]

why i measures the potential difference of galvanic cell by using potentiometer which have a voltage divider that supply galvanic cell with external potential opposing that of the galvanic cell and not using a voltmeter instead of that and directly measures the potential difference of galvanic cell !!
 

[Hunter2]

With this kind of procedure you dont have much lost of voltage because of resitance. A voltmeter has an internal resistance what can change your results.