Editing Avogadro constant (section)

=== First measurements ===
[[File:Johann Josef Loschmidt portrait plaque.jpg|right|thumb|Josef Loschmidt]]
The value of Avogadro's number (not yet known by that name) was first obtained indirectly by [[Johann Josef Loschmidt|Josef Loschmidt]] in 1865, by estimating the number of particles in a given volume of gas.<ref name=losch1865/> This value, the [[number density]] {{math|''n''{{sub|0}}}} of particles in an [[ideal gas]], is now called the [[Loschmidt constant]] in his honor, and is related to the Avogadro constant, {{math|''N''{{sub|A}}}}, by
: <math>n_0 = \frac{p_0N_{\rm A}}{R\,T_0},</math>
where {{math|''p''{{sub|0}}}} is the [[pressure]], {{math|''R''}} is the [[gas constant]], and {{math|''T''{{sub|0}}}} is the [[absolute temperature]]. Because of this work, the symbol {{math|''L''}} is sometimes used for the Avogadro constant,<ref name=bipm1971/> and, in [[German language|German]] literature, that name may be used for both constants, distinguished only by the [[units of measurement]].<ref name=virgo1933/> (However, {{math|''N''{{sub|A}}}} should not be confused with the entirely different [[Loschmidt constant]] in English-language literature.)

Perrin himself determined the Avogadro number, which he called "Avogadro's constant" (constante d'Avogadro), by several different experimental methods. He was awarded the 1926 [[Nobel Prize in Physics]], largely for this work.<ref name=oseen1926/>

The electric charge per [[Mole (unit)|mole]] of electrons is a constant called the [[Faraday constant]] and has been known since 1834, when [[Michael Faraday]] published [[Faraday's laws of electrolysis|his works on electrolysis]]. In 1910, [[Robert Millikan]] with the help of [[Harvey Fletcher]] obtained the first measurement of the [[elementary charge|charge on an electron]]. Dividing the charge on a mole of electrons by the charge on a single electron provided a more accurate estimate of the Avogadro number.<ref name=ebrit1974/>