Jen is conducting an experiment involving a device known as a pith-ball electroscope, shown below. The plastic ball of the electroscope hangs in midair and is uncharged. Charges cannot move along the silk thread that holds the ball in the air.
Jen completes the following steps in her experiment:

1- She collects three identical, uncharged rubber tubes.
2-She holds a magnet to the first tube for 5 minutes.
3-She submerges the second tube in a tank of water for 5 minutes.
4- She rubs the third tube with animal fur for 5 minutes.

Jen's teacher tells her that she can use the pith-ball electroscope to see if any of the rubber tubes are surrounded by an electric field. What should Jen do to test for an electric field surrounding each tube?