Moby electron: trapping a whale of a charge.

When a single electron hasn't the electric charge or mass to do the job, researchers may need to bring in reinforcements.

To measure a certain subtle effect that involves a moving electron, physicists have trapped and packed together 1,000 electrons to create a tiny, cold, dense ball of charge. This spherical droplet acts somewhat like a single particle with 1,000 times the charge and mass of an electron.

"We call it a kiloelectron," says Hans G. Dehmelt of the University of Washington in Seattle. By using such an electron ball, the researchers can magnify the effect they want to measure, making it readily detectable.

Dehmelt and Washington colleagues Richard K. Mittleman and Sander Kim describe their achievement in a paper to be published in Physical Review Letters.

Over the years, Dehmelt and his collaborators have focused on elucidating the structure--if any--of the electron. To obtain clues that the electron itself may be composed of tinier components, they have made extremely precise measurements of such characteristics as an electron's magnetism, or magnetic moment. These studies have generally involved a single electron trapped by a web of magnetic and electric fields inside a copper chamber.

However, an electron inside such a trap induces an electric charge in its copper surroundings, which in turn affects the electron's motion. Thus, any measurements of the electron's movements must be corrected for the interaction between the electron and its "image" charge.

"To obtain the correction, you have to make an independent measurement of this effect, and you cannot do this on a single electron," Dehmelt says.

The researchers succeeded in loading their trap with 1,000 electrons, confining them to a ball having an initial radius of less than 140 micrometers at a temperature of 4 kelvins. This is "only a few times larger than the closest confinement reported for an individual electron," the researchers report.

By using droplets containing different numbers of electrons, Dehmelt and his team obtained the data needed to determine the correction factor for a single electron. Now, the researchers want to make the same measurements at different magnetic fields. These data can then go into a new, more accurate determination of the electron's magnetic moment.