A very simple experiment shows the difference between an elastic and a rigid bandage. The experiment consists of placing the sheath of a tensiometer (for the measurement of arterial tension) on the lower third of the leg and then of raising the pressure to 80 mmHg (between the systolic and diastolic pressure. The foot must be placed in a hanging position (slight extension) and raised in dorsiflexion. It can be seen that the compression achieved using the rigid sheath (canvas) rises to double the pressure at its lowest. After several moments of this sort the pressure drops because the blood, lymph, and interstitial liquid are expelled from this segment of the member. The pressure differential is therefore 6-7 cmHg. And this is the differential required to eliminate oedema.