# Drift velocity of electron

Why it is not possible to measure the drift velocity for electron by timing their travel along a conductor? I know electron is very small and we cannot measure each electron time of collision but than how we measure drift velocity if we can't calculate time?

• It is really simple - you take one electron, paint it blue, and then time it as it goes down your wire. Oh, wait, we can't paint one blue and distinguish it from the rest of them... Sigh. Commented Nov 11, 2016 at 17:29
• @JonCuster With a brief explanation of why we can't paint one electron blue, that would be better suited as an answer than a comment.
– rob
Commented Nov 11, 2016 at 20:40

The mean drift velocity $v$ of an electron in a conductor can be easily measured by measuring the current density $j=I/A$, $I$ current, $A$ cross section area, and the electron density $n$ from a Hall effect measurement $R_H=\frac{1}{e n}$. Thus you get the mean drift velocity of the electrons $$v=\frac{j}{n e}$$