Hansjörg Scherer, Uwe Siegner
1. Introduction
In every aspect of our daily lives, we are sur-rounded by electricity – every household receives an electricity bill at regular intervals. However, the “product” we are charged for on this bill is not the electric current, but the electric energy supplied for use in lighting, heating, cooling and mechan-ical work. The physical quantity of “current” is defined as the amount of electric charge that flows through the cross-section of an electric conductor, divided by the length of an interval of time. In order to measure the electric current, and to define its corresponding physical quantity (the ampere), different effects of the electric current can be used, as history shows us. For example, if current is con-ducted through a metallic salt solution, the metal ions are discharged and the metal is separated at the cathode. Until the mid-20th century, the unit of electric current was defined on the basis of an electrolytic process of this kind. This so-called “international ampere” was also defined according to the 1898 laws of the then German Empire; the definition reads as follows:
"The ampere is the unit of electric current. It is realized by means of the unchanging electric current that, upon passing through an aqueous solution of silver nitrate, deposits 0.001118 grams of silver in one second.”