Schematic diagram of a <a href="https://en.wikipedia.org/wiki/Crookes_tube" class="extiw" title="en:Crookes tube">Crookes tube</a>.  This was a <a href="https://en.wikipedia.org/wiki/cold_cathode" class="extiw" title="en:cold cathode">cold cathode</a> discharge tube invented by <a href="https://en.wikipedia.org/wiki/William_Crookes" class="extiw" title="en:William Crookes">William Crookes</a>  and other physicists around the 1870s in which <a href="https://en.wikipedia.org/wiki/cathode_ray" class="extiw" title="en:cathode ray">cathode rays</a> (electrons) were discovered.  It consisted of a partially evacuated glass tube with two electrodes.  When a high potential of several thousand volts is applied between the electrodes, cathode rays are emitted by the negative electrode <i>(right)</i> and travel in straight lines through the tube. When they hit the glass wall of the tube <i>(left)</i> they cause it to glow or <a href="https://en.wikipedia.org/wiki/fluorescence" class="extiw" title="en:fluorescence">fluoresce</a>.   In this common example, a metal cross is placed in the tube in front of the cathode, which blocks the rays.  The shadow it throws on the glowing back wall demonstrates that the cathode rays travel in parallel lines.
created by <a href="https://en.wikipedia.org/wiki/User:Chetvorno" class="extiw" title="en:User:Chetvorno">Chetvorno</a>, rebuilt by <a href="https://en.wikipedia.org/wiki/User:Drondent" class="extiw" title="en:User:Drondent">Drondent</a>
created on 2008-03-05, updated on 2008-09-12 <br><small>uploaded on 2008-12-10</small>
own work by Chetvorno and rebuilt by Drondent <br><small><a class="external free" href="https://en.wikipedia.org/wiki/Image:Crookes_tube2_diagram.svg">http://en.wikipedia.org/wiki/Image:Crookes_tube2_diagram.svg</a> (english Wikipedia)</small>