Editing Oil drop experiment (section)

=== Comparison to modern values ===
Effective from the [[2019 revision of the SI]], the value of the elementary charge is ''defined'' to be exactly {{physconst|e}}.
Before that, the most recent (2014) accepted value<ref>{{cite web |url=http://physics.nist.gov/cuu/Constants/archive2014.html |title=2014 CODATA Values: Older values of the constants |date=25 June 2015| work=The NIST Reference on Constants, Units, and Uncertainty |publisher=[[National Institute of Standards and Technology|NIST]] |access-date=2019-08-19}}</ref> was {{val|1.6021766208|(98)|e=-19|u=[[Coulomb|C]]}}, where the (98) indicates the uncertainty of the last two decimal places. In his Nobel lecture, Millikan gave his measurement as {{val|4.774|(5)|e=-10|u=[[statcoulomb|statC]]}},<ref>{{cite speech | title = The electron and the light-quant from the experimental point of view | first = Robert A. | last = Millikan | author-link = Robert Millikan | date = May 23, 1924 | location = Stockholm | url = http://nobelprize.org/nobel_prizes/physics/laureates/1923/millikan-lecture.html | access-date = 2006-11-12}}</ref> which equals {{val|1.5924|(17)|e=-19|u=C}}. The difference is less than one percent, but is six times greater than Millikan's [[standard error]], so the disagreement is significant.

Using [[X-ray]] experiments, Erik Bäcklin in 1928 found a higher value of the elementary charge, {{val|4.793|0.015|e=-10|u=statC}} or {{val|1.5987|0.005|e=-19|u=C}}, which is within uncertainty of the exact value. [[Raymond Thayer Birge]], conducting a review of physical constants in 1929, stated "The investigation by Bäcklin constitutes a pioneer piece of work, and it is quite likely, as such, to contain various unsuspected sources of systematic error. If [... it is ...] weighted according to the apparent probable error [...], the weighted average will still be suspiciously high. [...] the writer has finally decided to reject the Bäcklin value, and to use the weighted mean of the remaining two values." Birge averaged Millikan's result and a different, less accurate X-ray experiment that agreed with Millikan's result.<ref>{{cite journal |last1=Birge |first1=Raymond T. |title=Probable Values of the General Physical Constants |journal=Reviews of Modern Physics |date=1 July 1929 |volume=1 |issue=1 |pages=1–73 |doi=10.1103/revmodphys.1.1|bibcode=1929RvMP....1....1B }}</ref> Successive X-ray experiments continued to give high results, and proposals for the discrepancy were ruled out experimentally. [[Sten von Friesen]] measured the value with a new [[electron diffraction]] method, and the oil drop experiment was redone. Both gave high numbers. By 1937 it was "quite obvious" that Millikan's value could not be maintained any longer, and the established value became {{val|4.800|0.005|e=-10|u=statC}} or {{val|1.6011|0.0017|e=-19|u=C}}.<ref>{{cite journal |last1=von Friesen |first1=Sten |title=On the values of fundamental atomic constants |journal=Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences|date=June 1937 |volume=160 |issue=902 |pages=424–440 |doi=10.1098/rspa.1937.0118|bibcode=1937RSPSA.160..424V |doi-access=free }}</ref>