Attempt from star

With Stern's experiment in 1920 , Otto Stern succeeded in measuring the speed of atoms directly for the first time .

Principle of the Stern experiment

A silver- coated platinum wire is located in a highly evacuated vessel . This is electrically heated so that the silver evaporates. The silver atoms move radially in all directions at a temperature-specific speed. A fine beam of silver atoms is focused through a narrow aperture , which would blacken the screen at point S ₀ when the apparatus is stationary .

To measure the atomic speed, the whole apparatus is rotated around the wire with the frequency . Even then, the orbit of an atom (viewed from the laboratory system) is straight. Viewed from the co-rotating reference system, however, the path appears curved, so the beam is apparently deflected around the distance . Depending on the direction of rotation, the screen is blackened at one point S ₁ or S ₂ . ${\displaystyle f}$ ${\displaystyle \Delta s}$

Test result

Darkness image of the attempt by Stern

The graphic shows the blackening image obtained in the experiment when the apparatus is first turned to the left and then to the right. The speeds measured in Stern's experiment agree well with the speeds predicted by the theory. The blurring of the two lines S ₁ and S ₂ shows that the atoms have different speeds. This velocity distribution was theoretically predicted as the Maxwell-Boltzmann distribution as early as the 19th century by James Clerk Maxwell and Ludwig Boltzmann .

Calculation of the atomic speed

In the co-rotating system, the speed component of a silver atom in the radial direction is given by the speed at which the atom emerges from the wire. The time from passing the screen to hitting the screen is therefore ${\displaystyle v}$ ${\displaystyle \Delta t}$

${\displaystyle \Delta t={\frac {l}{v}}}$.

The distraction results from the proportion ${\displaystyle \Delta s}$

${\displaystyle {\frac {\Delta s}{2\pi \cdot R}}={\frac {\Delta t}{T}}}$,

where the duration denotes a complete revolution of the apparatus. ${\displaystyle T={\frac {1}{f}}}$

It follows from this

${\displaystyle \Delta s={\frac {2\pi \cdot R}{T}}\cdot \Delta t={\frac {2\pi \cdot R\cdot l}{T\cdot v}}={\frac {2\pi \cdot R\cdot l\cdot f}{v}}}$

or.

${\displaystyle v={\frac {2\pi \cdot R\cdot l\cdot f}{\Delta s}}}$

Web links

• Experiment description and evaluation at student level ( LEIFI )