Direct Measurement of Thermal Molecular Velocities

O. Stern. Eine direkte Messung der thermischen Molekulargeschwindigkeit. Phys. Zeitschr. 21, p. 582, (1920).

Stern carried out a direct measurement of molecular velocities in the following manner. A slightly silvered platinum wire was heated in a vacuum by a current to such a degree that the silver coating melted, and the evaporating silver atoms scattered radially in all directions. Parallel to the heated wire there was placed a slit, beyond which a glass plate was placed. The slit admitted only a narrow beam of silver atoms, which were deposited in the form of a thin strip on the glass plate. The entire system, i.e. the wire, the slit, and the glass plate, was fastened to a common frame, which could be rotated rapidly about an axis passing through the middle of the wire and the slit. Owing to the fact that the molecules possess a certain finite velocity, they require some time to pass from the slit to the glass plate; the result of this is a displacement of the silver strip deposited on the glass relative to the direction of the wire, in the direction opposite to the rotation of the system. Knowing the speed of rotation, the angle of displacement, and the dimensions of the system, one can calculate the mean velocity of molecular motion. The numbers found by the author agree well with the theoretical ones.

S. Vavilov.