fine-structure-constant

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In physics, the fine-structure constant, also known as the Sommerfeld constant, commonly denoted by α (the Greek letter alpha), is a fundamental physical constant which quantifies the strength of the electromagnetic interaction between elementary charged particles.

It is a dimensionless quantity, independent of the system of units used, which is related to the strength of the coupling of an elementary charge e with the electromagnetic field, by the formula 4πε0ħcα = e2. 

The constant was named by Arnold Sommerfeld, who introduced it in 1916[2] when extending the Bohr model of the atom. α quantified the gap in the fine structure of the spectral lines of the hydrogen atom, which had been measured precisely by Michelson and Morley in 1887.[b]

Here is a scan giving the precise value of the Fine Structure Constant, scanned from The Planetary Scientist's Companion, by Lodders and Fegley, published by Oxford University Press, 1998, Table 1.5, page 5.