English: A Crookes tube to demonstrate the focusing and heating effect of cathode rays made by William Crookes and demonstrated by him at his lecture to the British Association for the Advancement of Science in Sheffield August 22, 1879. It consists of a glass bulb evacuated to a pressure of about 10⁻⁶ atm, with a spherical concave negative electrode or cathode (a) at the bottom, a small positive electrode or anode (P) at top, and a metal rod (b) whose tip is at the focus of the cathode. When a high voltage is applied to the tube, cathode rays (electrons) are emitted from the cathode at bottom. They are emitted perpendicularly to the cathode surface, since the electric field is normal to the surface. So all the electrons converge on the tip of the metal rod, and the kinetic energy of the fast moving electrons heats the rod white-hot.