The researchers noticed that, keeping the air flow from the compressor steady, the flame jet in the tube appeared to lengthen when the microwave power was increased. They set about trying to measure how much thrust was being produced, which proved difficult since the thousand-degree plasma jet would destroy a regular barometer. Instead, they settled on balancing a hollow steel ball on top of the tube, which could be filled with smaller steel beads to change its weight. At a certain weight, the thrust would counteract the gravitational forces pulling the ball down and begin lifting it off the tube, causing it to move and jump about, and the researchers used these measurements, minus the thrust contributed by the air compressor, to work out how hard their new plasma thruster was pushing.