Conductivity in the sense it is used in this article is the ability of a substance such as water to conduct electric current. It is expressed in terms of current per unit of applied voltage. It is the reciprocal of resistivity. Conductance is the reciprocal of resistance and is measured in siemens. It is therefore the ratio of the current through a substance to the potential difference at its ends. The tester described in this article is intended for assessing the quality of water, based on the acidity or alkalinity (pH), by means of a measurement of the conductance of the water.