Crookes tube. A Crookes tube, used to create cathode rays, made in the 1880s. This is a tube containing a near vacuum. A high voltage is applied across the two electrodes. Electrons naturally present are accelerated toward the anode (bottom), as they collide with gas molecules they produce positive ions that accelerate toward the cathode (top). When these strike the cathode they knock more electrons from the surface that fly off toward the anode. These are cathode rays. In this tube, a small sample of metal has been placed in the middle to study the effect of cathode rays upon it. This object is preserved at the Royal Institution, London.