<P> X-rays make up X-radiation, a form of electromagnetic radiation . Most X-rays have a wavelength ranging from 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz (3 × 10 Hz to 3 × 10 Hz) and energies in the range 100 eV to 100 keV . X-ray wavelengths are shorter than those of UV rays and typically longer than those of gamma rays . In many languages, X-radiation is referred to with terms meaning Röntgen radiation, after the German scientist Wilhelm Röntgen, who usually is credited as its discoverer, and who named it X-radiation to signify an unknown type of radiation . Spelling of X-ray (s) in the English language includes the variants x-ray (s), xray (s), and X ray (s). </P> <P> German physicist Wilhelm Röntgen is usually credited as the discoverer of X-rays in 1895, because he was the first to systematically study them, though he is not the first to have observed their effects . He is also the one who gave them the name "X-rays" (signifying an unknown quantity) though many others referred to these as "Röntgen rays" (and the associated X-ray radiograms as, "Röntgenograms") for several decades after their discovery and even to this day in some languages, including Röntgen's native German . </P> <P> X-rays were found emanating from Crookes tubes, experimental discharge tubes invented around 1875, by scientists investigating the cathode rays, which are 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 . Many of the early Crookes tubes undoubtedly radiated X-rays, because early researchers noticed effects that were attributable to them, as detailed below . Wilhelm Röntgen was the first to systematically study them, in 1895 . </P>

When was the term x ray first used
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