I was looking for the earliest experiment or the paper which shows the determination of the mass of proton. In NIST CODATA, the mass of proton is listed as "1.672 621 923 69 x 10$^{-27 }$kg". What type of measurements led to this number? I am not looking for the exact value but rather the experiments which allowed the measurement of proton's mass.

It is so easy to find the first paper on the mass of neutron but the reference to the earliest measurement of the mass of proton remains elusive. Google Scholar was tried with year limits, different names "hydrogen nucleus", "mass of hydrogen ion", mass of proton etc by limiting to 1950s but nothing relevant was found.

Note that the e/m of hydrogen "Kanalstrahlen" was measured in XLVII. On rays of positive electricity by J.J. Thomson in 1907 https://doi.org/10.1080/14786440709463633.

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    $\begingroup$ The original work appears to have been done by Wilhelm Wien (ca. 1898) who also developed mass spectrometry (charged particles in a magnetic field move in a circle with a radius that depends on mass). Thompson refined the method some 10 years later. $\endgroup$
    – sand1
    Nov 12 at 17:09
  • $\begingroup$ @sand1, Wien measured mass to charge ratios as far as I know. $\endgroup$
    – M. Farooq
    Nov 12 at 17:37
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    $\begingroup$ The quoted valued for the mass of a proton appears to be a quite recent estimate. An article on phys.org describes how this value was obtained using Penning Traps to compare the mass of a proton to the mass of a $^{12}C^{6+}$ isotope. The resulting (equivalent) measurement is given in atomic mass units rather than kilograms. $\endgroup$
    – nwr
    Nov 12 at 18:13

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