It represents a galvanic element similar to a battery - producing an electric current, measurable as an electric field on the surface of the concrete. This potential field can be measured with an electrode known as a half-cell. By making measurements over the whole surface, a distinction can be made between corroding and non - corroding locations. There are many scientific studies describing this method which has been known in the USA for more than 30 years. These form the basis for the successful practical application.