PH meter is an electric device that measures hydrogen-ion activity (acidity or alkalinity) in a solution.
Is basically a voltmeter attached to a PH responsive electrode and a reference (unvarying) electrode.
The PH responsive electrode is usually glass and the reference is usually a mercury-mercurous chloride (calomel) electrode. Although a silver-silver chloride is sometimes used.
When the two electrodes are immersed in a solution they act as a battery.
The glass electrode develops an electric potential (charge) that is directly related to the hydrogen-ion activity in the solution (59.2 millivolts per PH unit at 25C (77F) and the voltmeter measures the potential difference between the glass and the reference electrode