Editing Dalton (unit) (section)

=== Electron mass measurement ===
In practice, the atomic mass constant is determined from the [[electron rest mass]] ''m''{{sub|e}} and the [[electron relative atomic mass]] ''A''{{sub|r}}(e) (that is, the mass of electron divided by the atomic mass constant).<ref>{{cite journal |author-first1=Peter J. |author-last1=Mohr |author-first2=Barry N. |author-last2=Taylor |title=CODATA recommended values of the fundamental physical constants: 1998 |journal=[[Journal of Physical and Chemical Reference Data]] |volume=28 |issue=6 |pages=1713–1852 |bibcode=1999JPCRD..28.1713M |doi=10.1063/1.556049 |year=1999 |url=https://www.nist.gov/pml/div684/fcdc/upload/rmp1998-2.pdf |archive-url=https://web.archive.org/web/20171001122752/https://www.nist.gov/sites/default/files/documents/pml/div684/fcdc/rmp1998-2.pdf|archive-date=2017-10-01}}</ref> The relative atomic mass of the electron can be measured in [[cyclotron]] experiments, while the rest mass of the electron can be derived from other physical constants.
{{block indent|<math>m_{\rm u} = \frac{m_{\rm e}}{A_{\rm r}({\rm e})} = \frac{2R_\infty h}{A_{\rm r}({\rm e})c\alpha^2} ,</math>}}
{{block indent|<math>m_{\rm u} = \frac{M_{\rm u}}{N_{\rm A}} ,</math>}}
{{block indent|<math>N_{\rm A} = \frac{M_{\rm u} A_{\rm r}({\rm e})}{m_{\rm e}} = \frac{M_{\rm u} A_{\rm r}({\rm e})c\alpha^2}{2R_\infty h} ,</math>}}
where ''c'' is the [[speed of light]], ''h'' is the [[Planck constant]], ''α'' is the [[fine-structure constant]], and ''R''{{sub|∞}} is the [[Rydberg constant]].

As may be observed from the old values (2014 CODATA) in the table below, the main limiting factor in the precision of the Avogadro constant was the uncertainty in the value of the [[Planck constant]], as all the other constants that contribute to the calculation were known more precisely.
{| class="wikitable"
|- style="line-height:133%"
! Constant
! Symbol
! 2014 [[Committee on Data for Science and Technology|CODATA]] values
! Relative{{br}}standard{{br}}uncertainty
! Correlation{{br}}coefficient{{br}}with ''N''{{sub|A}}
|-
| [[Proton–electron mass ratio]]
| align="center" |''m''{{sub|p}}/''m''{{sub|e}}
| {{val|1836.15267389|(17)}}
| align="center" |{{val|9.5|e=-11}}
| {{val|−0.0003}}
|-
| [[Molar mass constant]]
| align="center" |''M''{{sub|u}}
| 1 g/mol
| align="center" |'''0''' (defined)
| —
|-
| [[Rydberg constant]]
| align="center" |''R''{{sub|∞}}
| {{val|10973731.568508|(65)|u=m-1}}
| align="center" |{{val|5.9|e=-12}}
| −0.0002
|-
| [[Planck constant]]
| align="center" |''h''
| {{val|6.626070040|(81)|e=-34|u=J.s}}
| align="center" |{{val|1.2|e=-8}} 
| −0.9993
|-
| [[Speed of light]]
| align="center" |''c''
| {{val|299792458|u=m|up=s}}
| align="center" |'''0''' (defined)
| —
|-
| [[Fine structure constant]]
| align="center" |''α''
| {{val|7.2973525664|(17)|e=-3}}
| align="center" |{{val|2.3|e=-10}}
| 0.0193
|-
| [[Avogadro constant]]
| align="center" |''N''{{sub|A}}
| {{val|6.022140857|(74)|e=23|u=mol-1}}
| align="center" |{{val|1.2|e=-8}} 
| 1
|-
|}

The power of having defined values of [[universal constant]]s as is presently the case can be understood from the table below (2018 CODATA).

{| class="wikitable"
|- style="line-height:133%"
! Constant
! Symbol
! 2018 [[Committee on Data for Science and Technology|CODATA]] values<ref>{{Cite web|url=https://physics.nist.gov/cuu/Constants/bibliography.html|title=Constants bibliography, source of the CODATA internationally recommended values|website=The NIST Reference on Constants, Units, and Uncertainty|access-date=4 August 2021}}</ref>
! Relative{{br}}standard{{br}}uncertainty
! Correlation{{br}}coefficient{{br}}with ''N''{{sub|A}}
|-
| [[Proton–electron mass ratio]]
| align="center" |''m''{{sub|p}}/''m''{{sub|e}}
| {{val|1836.15267343|(11)}}
| align="center" |{{val|6.0|e=-11}}
| —
|-
| [[Molar mass constant]]
| align="center" |''M''{{sub|u}}
| {{val|0.99999999965|(30)|u=g|up=mol}}
| align="center" |{{val|3.0|e=-10}}
| —
|-
| [[Rydberg constant]]
| align="center" |''R''{{sub|∞}}
| {{val|10973731.568160|(21)|u=m-1}}
| align="center" |{{val|1.9|e=-12}}
| —
|-
| [[Planck constant]]
| align="center" |''h''
| {{val|6.62607015|e=-34|u=J.s}}
| align="center" |'''0''' (defined)
| —
|-
| [[Speed of light]]
| align="center" |''c''
| {{val|299792458|u=m|up=s}}
| align="center" |'''0''' (defined)
| —
|-
| [[Fine structure constant]]
| align="center" |''α''
| {{val|7.2973525693|(11)|e=-3}}
| align="center" |{{val|1.5|e=-10}}
| —
|-
| [[Avogadro constant]]
| align="center" |''N''{{sub|A}}
| {{val|6.02214076|e=23|u=mol-1}}
| align="center" |'''0''' (defined)
| —
|-
|}