The paper considers the effect of a thin conducting vacuum chamber on the movement of the plasma column in a tokamak during plasma compression in the direction of the major radius. It is shown that a sort of frictional force acts on the plasma column from the direction of the vacuum chamber. The maximum value of this force per unit length can be considerable: Fmax ≅ J2/(ac2), where J is the plasma current and a is the distance from the plasma centre to the side wall of the chamber. A study of the dynamics of the plasma column motion shows that in the presence of a vacuum chamber the displacement velocity of the column is bounded by the value V* ≅ c2/(2 πσΔ), where Δ is the thickness and σ the conductivity of the chamber wall. If the velocity rises above the critical value, the compression is accompanied by the development of self-oscillations.