This can be further simplified to formula_4. Multiplying both sides by formula_5 gives formula_6. The concept of radian measure, as opposed to the degree of an angle, is normally credited to Roger Cotes in 1714. He described the radian in everything but name, and he recognized its naturalness as a unit of angular measure. Prior to the term "radian" becoming widespread, the unit was commonly called "circular measure" of an angle. The idea of measuring angles by the length of the arc was already in use by other mathematicians. For example, al-Kashi (c. 1400) used so-called "diameter parts" as units where one diameter part was radian and they also used sexagesimal subunits of the diameter part. The term "radian" first appeared in print on 5 June 1873, in examination questions set by James Thomson (brother of Lord Kelvin) at Queen's College, Belfast. He had used the term as early as 1871, while in 1869, Thomas Muir, then of the University of St Andrews, vacillated between the terms "rad", "radial", and "radian". In 1874, after a consultation with James Thomson, Muir adopted "radian". The name "radian" was not universally adopted for some time after this. "Longmans' School Trigonometry" still called the radian "circular measure" when published in 1890. The International Bureau of Weights and Measures and International Organization for Standardization specify rad as the symbol for the radian. Alternative symbols used 100 years ago are c (the superscript letter c, for "circular measure"), the letter r, or a superscript , but these variants are infrequently used as they may be mistaken for a degree symbol (°) or a radius (r). So, for example, a value of 1.2 radians would most commonly be written as 1.2 rad; other notations include 1.2 r, 1.2, 1.2, or 1.2. As stated, one radian is equal to 180/ degrees. Thus, to convert from radians to degrees, multiply by 180/. For example:  Conversely, to convert from degrees to radians, multiply by /180. For example:  formula_13  Radians can be converted to turns (complete revolutions) by dividing the number of radians by 2. The length of circumference of a circle is given by formula_14, where formula_15 is the radius of the circle.