the remainder being distributed to signatories of the Metre Convention . [ 14 ] In 1889 there was no generally accepted theory regarding the nature of light but by 1960 the wavelength of specific light spectra could give a more accurate and reproducible value than a prototype metre. In that year the prototype metre was replaced by a formal definition which defines the metre in terms of the wavelength of specified light spectra. By 1983 it was accepted that the speed of light was constant and that this constant provided a more reproducible procedure for measuring length. Therefore the metre was redefined in terms of the speed of light. These definitions give a much better reproducibility and also allow anyone, anywhere with a suitably equipped laboratory, to make a standard metre. [ 15 ] None of the other base units rely on a prototype – all are based on phenomena that are directly observable and had been in use for many years before formally becoming part of the metric system. The second first became a de facto base unit within the metric system when, in 1832, Carl Friedrich Gauss used it, the centimetre and the gram to express the units associated with values of absolute measurements of the Earth's magnetic field . [ 16 ] The second, if based on the Earth's rotation , is not a constant as the Earth's rotation is slowing down—in 2008 the solar day was 0.002 s longer than in 1820. [ 17 ] This