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 4 πε 0 ħcα = e. Its numerical value is approximately 0.007 297 352 5643 ≃ ⁠ 1 / 137.035 999 177 ⁠, with a relative uncertainty of 1.6 × 10. ‍