X-rays were found emanating from Crookes tubes, experimental discharge tubes invented around 1875, by scientists investigating the cathode rays, that is energetic electron beams, that were first created in the tubes.
Crookes tubes created free electrons by ionization of the residual air in the tube by a high DC voltage of anywhere between a few kilovolts and 100 kV. This voltage accelerated the electrons coming from the cathode to a high enough velocity that they created X-rays when they struck the anode or the glass wall of the tube.
On November 8, 1895, German physics professor Wilhelm Röntgen stumbled on X-rays while experimenting with Lenard and Crookes tubes and began studying them.
He wrote an initial report “On a new kind of ray: A preliminary communication” and on December 28, 1895. Röntgen referred to the radiation as “X”, to indicate that it was an unknown type of radiation.