# The determination of the charge and mass of the electron

## Robert A. Millikan

Thomson's experiment revealed the ratio of mass and charge $e$ of the electron:
$r$ $=$ $\frac{m_{e^-}}{e}$
It was impossible so far to know these two physical parameter separately.

## The experiment

Millikan imagined a genial experiment to detect the charge of the electron:

Small drops of oils are formed by a sprayer, and some of them attach themselves to electrons that have been produced by the action of xrays on molecules.
The experimenter measures first the rate at which a drop falls in the earth's gravitational field.
By charging the plates, drops carrying one additional electron fall with a different speed and so the charge of this electron can be calculated.

$e = -1,6 \cdot 10^{-19} C$

The charge $e$ of one electron is called elementary negative charge.
A simple calculation gives then the electron mass:
$m_{e^-}$ $=$ $r \cdot e$

$m_{e^-}$ $=$ $9,1 \cdot 10^{-31} kg $

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