Roentgen (unit) Roentgen (unit) The roentgen or röntgen () (symbol R) is a legacy unit of measurement for the exposure of X-rays and gamma rays. It is defined as the electric charge freed by such radiation in a specified volume of air divided by the mass of that air. In 1928 it was the first international measurement quantity for ionising radiation to be defined for radiation protection, and was an easily replicated method of measuring air ionization directly by using an ion chamber. It is named after the German physicist Wilhelm Röntgen, who discovered X-rays. Although relatively easy to measure, it had
Invention (musical composition) Bach adapted and modified the form to what is considered to be a formal invention. Bach wrote 15 inventions (BWV 772–786) as exercises for his son, Wilhelm Friedemann Bach. Bach later wrote a set of 15 three-part inventions, called sinfonias (BWV 787–801). Invention (musical composition) In music, an invention is a short composition (usually for a keyboard instrument) with two-part counterpoint. (Compositions in the same style as an invention but using three-part counterpoint are known as "sinfonias." Some modern publishers call them "three-part inventions" to avoid confusion with symphonies.) Well-known are the fifteen inventions that make up the first half