Editing Cathode ray (section)

===Cathode rays===
[[File:Ampola de Crookes.gif|thumb|A beam of cathode rays being bent by a magnetic field. Cathode rays are normally invisible; the path of this beam is revealed by having it strike a card with a fluorescent coating]] 
By the 1870s, British physicist [[William Crookes]] and others were able to evacuate tubes to a lower pressure, below 10<sup>−6</sup> atm. These were called [[Crookes tube]]s. Faraday had been the first to notice a dark space just in front of the cathode, where there was no luminescence. This came to be called the "cathode dark space", "Faraday dark space" or "Crookes dark space". Crookes found that as he pumped more air out of the tubes, the Faraday dark space spread down the tube from the cathode toward the anode, until the tube was totally dark. But at the anode (positive) end of the tube, the glass of the tube itself began to glow.

What was happening was that as more air was pumped from the tube, the electrons knocked out of the cathode when positive ions struck it could travel farther, on average, before they struck a gas atom. By the time the tube was dark, most of the electrons could travel in straight lines from the cathode to the anode end of the tube without a collision. With no obstructions, these low mass particles were accelerated to high velocities by the voltage between the electrodes. These were the cathode rays.

When they reached the anode end of the tube, they were traveling so fast that, although they were attracted to it, they often flew past the anode and struck the back wall of the tube. When they struck atoms in the glass wall, they excited their orbital electrons to higher [[energy level]]s. When the electrons returned to their original energy level, they released the energy as light, causing the glass to [[fluoresce]], usually a greenish or bluish color. Later researchers painted the inside back wall with fluorescent chemicals such as [[zinc sulfide]], to make the glow more visible.

Cathode rays themselves are invisible, but this accidental fluorescence allowed researchers to notice that objects in the tube in front of the cathode, such as the anode, cast sharp-edged shadows on the glowing back wall. In 1869, German physicist [[Johann Hittorf]] was first to realize that something must be traveling in straight lines from the cathode to cast the shadows. [[Eugen Goldstein]] named them ''cathode rays'' (German ''Kathodenstrahlen'').