The Stern experiment is a physical experiment first posed by the German physicist Otto Stern in 1920 and one of the first practical proofs of the consistency of the molecular-kinetic theory of the structure of matter . In it, the rates of thermal motion of molecules were directly measured and the presence of the distribution of gas molecules by velocities was confirmed.
To conduct the experiment, Stern prepared a device consisting of two cylinders of different radius, whose axis coincided and a platinum wire with a deposited layer of silver was located on it. A sufficiently low pressure was maintained in the space inside the cylinders by means of continuous pumping of air. When electric current was passed through the wire, the melting point of silver was reached, due to which silver began to evaporate and silver atoms flew to the inner surface of the small cylinder uniformly and rectilinearly at a speed determined by the heating temperature of the platinum wire, that is, the melting point of silver. A narrow gap was made in the inner cylinder through which the atoms could fly further unhindered. The walls of the cylinders were specially cooled, which contributed to the subsidence of atoms falling on them. In this state, a fairly clear narrow strip of silver plaque was formed on the inner surface of the large cylinder, located directly opposite the slit of the small cylinder. Then the whole system began to rotate with a certain sufficiently large angular velocity . In this case, the plaque was shifted in the direction opposite to the direction of rotation, and lost its sharpness. By measuring the offset the darkest part of the strip from its position when the system was at rest, Stern determined the flight time, after which he found the speed of movement of the molecules:
Where - strip offset Is the distance between the cylinders, and - the speed of the points of the outer cylinder.
The speed of motion of silver atoms (584 m / s) found in this way coincided with the speed calculated according to the laws of molecular kinetic theory, and the fact that the resulting strip was smeared testified to the fact that the speeds of the atoms are different and distributed according to some law - Maxwell’s distribution law : atoms moving faster were displaced relative to the strip obtained at rest by shorter distances than those moving slower. Moreover, the experiment provided only approximate information about the nature of the Maxwell distribution; more accurate experimental confirmation dates back to 1930 ( Lammert's experiment ).
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