To measure the cell potential of a galvanic cell, we can either use a voltmeter or a potentiometer. The former is more common, less expensive, but gives out measurements a bit lower than the actual max potential; whereas the latter is more expensive, but provides much more accurate max cell potential measurements.
A voltmeter measures the potential by drawing current produced by the reactions through a known high resistance. Note that since resistors emit heat, some of the electrical energy is lost as heat, so the voltmeter measurement will always be slightly lower than the maximum cell potential.
For more accurate measurements, we use a potentiometer. During measurements, we adjust the voltage until no current flows through the cell circuit. Cell potential is equal in magnitude and opposite in sign to the voltage setting of the potentiometer. Potentiometer measurement represents the maximum cell potential because no energy is wasted in heating the wire / resistor.