There are two parallel disks in a vessel with a gas whose pressure can be changed. One disk hangs on an elastic thread, the other rotates at a constant angular velocity. Twist angle of the first disk at pressure P1 is equal to φ1. As the gas pressure increases, the angle of twisting of the thread first increases, and then, having reached the value φ2, it ceases to depend on the gas pressure. Explain this effect. How does the thread twist angle depend on the gas pressure at φ≪φ2?