Now consider a portion of a cylindric film of length x terminated by two equal disks of radius r and containing a certain volume of air.
Now let us consider a cylindric film contained between two equal fixed disks A and B, and let a third disk, C, be placed midway between.
Hence if the length of the cylindric film is less than its circumference, it is in stable equilibrium.
But if the length of the cylindric film is greater than its circumference, and if we suppose the disk C to be placed midway between A and B, and to be moved towards A, the pressure on the side next A will diminish, and that on the side next B will increase, so that the resultant force will tend to increase the displacement, and the equilibrium of the disk C is therefore unstable.
Hence the equilibrium of a cylindric film whose length is greater than its circumference is unstable.
