Most people, including physicists , are probably not aware of how the voltmeter in a lab or the battery in a cell phone is calibrated. Both of those activities, and many others, depend crucially on the successful dissemination of electrical units based on the Système International. Standards for electrical units have a venerable history that dates back to fundamental experiments—for example, tests of Ampère’s law. Today’s electrical standards, though, are being challenged by modern work based on quantum laws and devices that did not exist when the SI was established in 1960.
In theory, electrical units are all based on the force between two current-carrying wires. In reality, the present system of electrical units is based on two inconvenient and challenging physics experiments. The unit of current is defined by a modern version of the Ampère experiment that uses a device called the watt balance (see figure 1). The...