What is a Cathode ray gun?

basically JJ Thomson wanted to find out what a cathode ray was made of. Cathode tubes are a tube with wires in them. Scientists discovered that if the created a vacuum in the tube and sent a charge through the wires, it glowed. JJ Thomson set up a cathode ray and placed magnets on either side. This deflected the ray so the tube didn't glow. This meant the rays were negatively charged which meant they were made of negatively charged particles or electrons.