A gas discharge tube, as used by Thomson for electron detection — He used tubes in which the cathode rays were deflected magnetically as well as those, as shown here, in which the electrons were deflected electrically. The cathode ray tube shown here is a replica in the Cavendish Laboratory at Cambridge University.
Photo Credit: By Rolf Kickuth - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=103442270