Counting electrons for a new standard - new standard for measuring capacitance - Brief Article

Science News, April 9, 1994 by Ivars Peterson

Counting electrons for a new standard

Nearly every electronic circuit in a stereo, computer, or scientific instrument requires at least one capacitor. This crucial component- often just a small ceramic disk or cylinder with two wires sticking out of it - stores electrical charge. In its simplest form, the device is nothing more than a pair of metal plates separated by a thin layer of electrically insulating material. Incorporated into circuits that handle changing voltages and currents, capacitors can shape, filter, shunt aside, or block electrical signals.

Measured in farads, capacitance represents the amount of stored charge per volt. Scientists and engineers have a variety of methods for determining capacitance, but the most accurate techniques are cumbersome, complicated, and inconvenient. Now, researchers at the National Institute of Standards and Technology (NIST) in Boulder, Colo. are in the midst of a program to develop a new standard for measuring capacitance based on fundamental physics. "We hope to come up with something that's simpler and more reliable," says NIST's John M. Martinis.

At the heart of the new technique lies the ability to count individual electrons and pump them through a circuit to charge up a standard capacitor with a known number of electrons. So far, Martinis and his colleagues have demonstrated that their microscopic electron pump works with sufficient accuracy to meet their needs. "We've proven that the technology works -- that we can control electrons one by one," Martinis says.

The trickiest part, however, lies ahead. "The main unknown now is building the capacitor where we're going to store those electrons," Martinis says. "We need a new kind of capacitor, one that is very stable. This really hasn't been looked at before."