Hertz radians per second. Thus a disc rotating at 60 revolutions per minute (rpm) is said to be rotating at either 2 rad/s "or" 1 Hz, where the former measures the angular velocity and the latter reflects the number of "complete" revolutions per second. The conversion between a frequency "f" measured in hertz and an angular velocity "ω" measured in radians per second is The hertz is named after the German physicist Heinrich Hertz (1857–1894), who made important scientific contributions to the study of electromagnetism. The name was established by the International Electrotechnical Commission (IEC) in 1930. It was adopted by
Unit of length in vacuum during a time interval of seconds." It is approximately equal to . Other units are derived from the metre by adding prefixes from the table below: For example, a kilometre is . In the Centimetre–gram–second system of units, the basic unit of length is the centimetre, or of a metre. Other non-SI units are derived from decimal multiples of the metre. The basic unit of length in the Imperial and U.S. customary systems is the yard, defined as exactly by international treaty in 1959. Common Imperial units and U.S. customary units of length include: In addition, the following