History
The roentgen has its roots in the Villard unit defined in 1908 by the American Roentgen Ray Society as "the quantity of radiation which liberates by ionisation one esu of electricity per cm3 of air under normal conditions of temperature and pressure." Using 1 esu ≈ 3.33564×10−10 C and the air density of ~1.293 kg/m³ at 0°C and 101kPa, this converts to 2.58×10−4 C/kg, which is the modern value given by NIST.
1 esu/cm3 × 3.33564×10-10 C/esu × 1,000,000 cm3/m3 ÷ 1.293 kg/m3 = 2.58×10-4 C/kg
This definition was used under different names (e, R, and German unit of radiation) for the next 20 years. In the meantime, the French Roentgen was given a completely different definition which amounted to 0.444 German R.
Read more about this topic: Roentgen (unit)
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