There have been follow-ups on the claims that this force can be produced in a full vacuum, meaning it is an unknown anti-gravity force, and not just the more well known ion wind. As part of a study in 1990, U.S. Air Force researcher R. L. Talley conducted a test on a Biefeld–Brown-style capacitor to replicate the effect in a vacuum. Despite attempts that increased the driving DC voltage to about 19 kV in vacuum chambers up to 10 torr, Talley observed no thrust in terms of static DC potential applied to the electrodes. In 2003, NASA scientist Jonathan Campbell tested a lifter in a vacuum at 10 torr with a voltage of up to 50 kV, only to observe no movement from the lifter. Campbell pointed out to a Wired magazine reporter that creating a true vacuum similar to space for the test requires tens of thousands of dollars in equipment.