Is the tube in the JJ Thomson experiment completely evacuated?

No, not completely evaluated. The pressure was set to 0.0001 mm of Hg. And electrons eject out only from cathode because cathode is negatively charged and has an excess of electrons, so it can eject electrons. read more

This discharge tube was later slightly modified by J.J.Thomson. When high voltage was applied between the cathode and the anode with a small hole at the center of a partially evacuated tube at a pressure of 0.01mm of Hg, a bright spot of light was formed on the zinc sulfide screen kept at the opposite end of the discharge tube. read more

the image showing a simple set up of JJ Thompson experiment. With the result of JJ Thompson experiment, the nature of electron was established. In his earlier experiment, JJ Thompson could not get much as there was much air in the cathode tube, but he created the vacuum and evacuated almost all of the air and applied the electric field across the cathode tube. read more