PARALLEL LINES

摘要

The ampere, the SI unit of electric current, currently has the most awkward definition of ail: "The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 × 10~(-7) newton per metre of length." But that's an impossible measurement to make. Fortunately, the new definition simply requires the counting of electrons. The new amp will be equal to 1.6 × 10~(19) electrons passing per second. Counting them can be done with an electron pump, a microscopic device that can hold a single electron and control its motion by changing the voltages on a set of electrodes. "It's like a lock on a canal," says Jonathan Williams at the UK National Physics Laboratory (NPL) in Teddington. "But in the electron pump, the gates open and close a billion times a second."