English: Drawing of a Crookes tube which demonstrated magnetic deflection of cathode rays, built by British physicist William Crookes around 1879. The tube was evacuated to a pressure of about 10^-6 to 10^-7 atmosphere. When a high DC voltage of several thousand volts or more was applied between the electrodes (N,P) in the tube, electrons (cathode rays) radiated from the negative terminal or cathode (N) on the left. The horizontal slit (e) in the metal divider (c,d) inside the tube allowed a thin beam to pass through. The horseshoe shaped magnet below the tube created a horizontal magnetic field in the center of the tube. When the electrons passed through it, the Lorentz force of the magnetic field on the moving electrons bent the beam (g) downward. When the magnet was removed, the electrons continued in a straight path (f) down the tube. The electrons themselves were invisible, but the divider was painted with a chemical, such as zinc sulfide, which fluoresced when struck by the electrons, which made the path of the beam visible. This experiment, which proved that cathode rays obeyed Faraday's law like electric currents in wires, was important in establishing that cathode rays, that is electrons, were the particles responsible for electricity.