Module 2 : Electrostatics
Lecture 6 : Quantization Of Charge
Once the mass is determined, the droplet is subjected to the electric field such that the droplet starts moving upward. In this situation, the air resistance (which is opposite to velocity direction) as well as the weight of the droplet balances the electric force and the droplet acquires a new terminal speed $v$.
   

$ qE= mg + 6\pi\eta av$
$\displaystyle{ q= \frac{6\pi\eta a}{E}(v+v_0)}$

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By measuring the time of fall between two positions the terminal velocity is determined. It is found that the charge on the droplet is an integral multiple of an elementary charge. The value of the elementary charge determined by Millikan was $1.592\times 10^{-19}$coulomb. The currently accepted value of the charge of an electron is $-1.602\times 10^{-19}$C. The experiment shows :

Electric charge exists in basic natural units and the value of the basic unit is equal to the magnitude of the charge of an electron

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