text: This constant times two is in the denominator of the expression for the magnetic flux quantum.
full_question: This constant times two is in the denominator of the expression for the magnetic flux quantum. This quantity squared is in the numerator of the expression for the fine structure constant. This value was incorrectly estimated in an experiment due to an error in the value for the viscosity of air; in that experiment, particles were suspended in the air using an E-field. This quantity, which is equal to Faraday's constant divided by Avogadro's number, was calculated in Robert Millikan's oil drop experiment. For 10 points, name this quantity equal to approximately "1.60 times 10 to the negative 19" coulombs which is the charge of a proton.
first_sentence: This constant times two is in the denominator of the expression for the magnetic flux quantum.
raw_answer: elementary charge [or fundamental charge; or charge of an electron; or charge of a proton until "proton" is read