The magnitude expresses the energy released by an earthquake. In 1935 Charles Francis Richter developed a method of measuring the strength of earthquakes. He compared the amplitudes of seismographs by converting them to a standard distance of 100 km from the epicentre. On the Richter scale, the unit of magnitude has a logarithmic structure, which means that each step represents a ten-fold increase in amplitude. Because of its logarithmic structure, the Richter scale is open-ended, having no defined upper limit. To date an earthquake with a magnitude of 9 or over has never occurred because rocks burst before such high energy levels can build up.