Charles Francis Richter, not surprisingly, is dead. Were he still alive, he would have been 100 today, since this is the centenary of his birth on April 26th, 1900.
Richter made his name in earthquakes. Before his time, the intensity or otherwise of an earthquake, insofar as it was quantified at all, was measured on one of a number of subjective scales, based on observations of the local damage.
The one most widely used was that devised in 1902 by Giuseppe Mercalli: it was, in fact, not dissimilar in concept to the Beaufort Scale of Wind Force used by meteorologists, with each "strength" on the Mercalli Scale being defined in terms of its observed effect on the environment, both structural and human.
The disadvantage of the Mercalli Scale, however, was that it did not provide a unique figure for a given seismic disturbance: the "strength" varied depending on the distance of the epicentre from the observation point.
Richter devised his scale in 1935 while working at the California Institute of Technology, and it aimed to be a measure of the total energy released by a given earthquake, this in turn deduced from seismographic readings.
His ambitions at the time were modest: he had hoped to construct a method for categorising earthquakes into a few broad bands, but in the event his scale was so good that the strength of a tremor could in due course be specified accurately to the nearest tenth magnitude. And it had the distinct advantage that while an earthquake might have an infinite number of Mercalli "strengths", it had only one Richter magnitude.
For practical purposes the scale runs from 0 to 9, although in theory there is no upper limit. But it is not a scale of equal increments, as is, for example, a thermometer scale, and there is no such unit as a "Richter" in the sense that we have "degrees Celsius" or "miles per hour".
On the contrary, an increase of one magnitude signifies an earthquake about 30 times as great as that corresponding to the lower number. It is for this reason that the magnitude of an earthquake is never described as, say, "5.4 Richters", but rather, in somewhat convoluted terms, as "measuring 5.4 on the Richter Scale"
The most powerful earthquake ever known, the Lisbon earthquake of November 1st, 1755, is believed to have had a magnitude of about 8.9. The best-remembered tremor, on the other hand, is probably that of April 18th, 1906, which destroyed most of the city of San Francisco and would probably have registered about 8.3 on Richter's scale.