The Cp/Cv tube comes with corks for either end of the tube and a metal ball of the same diameter as the inside of the tube. This ensures that air can not slip around the ball while the ball is inside the tube. Therefore, if one end of the tube is capped while the ball is inside, the ball cannot move due to constant and equal pressure above and below the ball. The temperature is equal on both sides of the ball and no air can escape from one side of the ball to the other, so the volume of the plugged region must remain constant. If the system is shocked with, say, a hit to the bottom of the tube, the ball will undergo heavily damped oscillations around the equilibrium point.
The Cp/Cv tube comes with corks for either end of the tube and a metal ball of the same diameter as the inside of the tube. This ensures that air can not slip around the ball while the ball is inside the tube. Therefore, if one end of the tube is capped while the ball is inside, the ball cannot move due to constant and equal pressure above and below the ball. The temperature is equal on both sides of the ball and no air can escape from one side of the ball to the other, so the volume of the plugged region must remain constant. If the system is shocked with, say, a hit to the bottom of the tube, the ball will undergo heavily damped oscillations around the equilibrium point.