We all know what a kilogram is, right? It’s the mass of a bag of sugar. Thank goodness for the bag of sugar. Without it we’d be forced to imagine 1/7000th of an African bull elephant or 1/14560th of a double decker bus or God only knows what fraction of the weight of Wales.
Having a clear visualisation of the kilogram is important for all sorts of reasons, not least because it’s one of the seven SI units, the fundamental alphabet of symbols which can be combined in a variety of ways to express any physical quantity. Speed, for instance, is measured in metres per second, whereas force can be expressed in terms of kilogram metres per second squared.
The kilogram was accepted into this metrological pantheon alongside the metre 125 years ago today, when the value of both units was defined at the first General Conference on Weights and Measures organised by the BIPM. As a consequence of that meeting, three identical kilograms were cast by the firm Johnson Matthey out of a mass of platinum-iridium alloy. This alloy was chosen because of its density and chemical stability, meaning that the kilograms would be both small and resistant to rust. One of the kilograms was kept at the BIPM offices in Sevres, and two were sent for safekeeping in America. Rumours that a further three were subsequently distributed to the elven-kings under the sky have no foundation in truth.