Stern-Zartman experiment[′stərn ′zärt·mən ik‚sper·ə·mənt]
(in Russian, called the Stern experiment), an experimental determination of the speeds of gas molecules in thermal motion. It was first carried out by O. Stern in 1920. The Stern-Zartman experiment confirmed the validity of the foundations of the kinetic theory of gases.
The gas studied in the experiment was a rarefied silver vapor, which was obtained by vaporizing a silver coating from a platinum wire heated by an electric current. The wire was placed in a vessel from which air had been evacuated, so that the silver atoms freely escaped from the wire in all directions. To obtain a narrow beam of escaping atoms, a barrier with a slit, through which the atoms impinged on a brass plate maintained at room temperature, was set up in the path of the escaping atoms. The silver atoms were deposited on the plate as a narrow band, forming a silver image of the slit. A special device was used to set the entire apparatus in rapid rotation about an axis parallel to the plane of the plate. As a result of the rotation of the apparatus, the atoms impinged on another site on the plate because the plate shifted as the atoms traversed the distance l from the slit to the plate (see Figure 1).
The shift of the plate increases as the angular velocity co of the apparatus increases and decreases as the speed v of the silver atoms increases. If ω and l are known, v may be determined. Since the atoms move with different speeds, the band spreads and becomes broader as the apparatus rotates. The density of the silver deposit at a given point of the band is proportional to the number of atoms moving with a given speed. The maximum density of the deposit corresponds to the most probable velocity of the atoms.
The values obtained in the Stern-Zartman experiment for the most probable velocity were in good agreement with the theoretical values obtained on the basis of the Maxwellian distribution of molecules over velocities.