The primary determination of an electron's mass comes from measuring the ratio of the mass to that of a nucleus, so that the result is obtained in u (atomic mass units). The conversion factor to MeV is more uncertain than the mass of the electron in u; indeed, the recent improvements in the mass determination are not evident when the result is given in MeV. In this datablock we give the result in u, and in the following datablock in MeV.

VALUE ($ 10^{-6} $ u) DOCUMENT ID TECN  COMMENT $548.579909065$ $\pm0.000000016$ TIESINGA 2021 RVUE 2018 CODATA value • • We do not use the following data for averages, fits, limits, etc. • • $548.579909070$ $\pm0.000000016$ MOHR 2016 RVUE 2014 CODATA value $548.57990946$ $\pm0.00000022$ MOHR 2012 RVUE 2010 CODATA value $548.57990943$ $\pm0.00000023$ MOHR 2008 RVUE 2006 CODATA value $548.57990945$ $\pm0.00000024$ MOHR 2005 RVUE 2002 CODATA value $548.5799092$ $\pm0.0000004$ 1 BEIER 2002 CNTR Penning trap $548.5799110$ $\pm0.0000012$ MOHR 1999 RVUE 1998 CODATA value $548.5799111$ $\pm0.0000012$ 2 FARNHAM 1995 CNTR Penning trap $548.579903$ $\pm0.000013$ COHEN 1987 RVUE 1986 CODATA value 1  BEIER 2002 compares Larmor frequency of the electron bound in a ${}^{12}\mathrm {C}{}^{5+}$ ion with the cyclotron frequency of a single trapped ${}^{12}\mathrm {C}{}^{5+}$ ion. 2  FARNHAM 1995 compares cyclotron frequency of trapped electrons with that of a single trapped ${}^{12}\mathrm {C}{}^{6+}$ ion.

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