Abstract: Plethysmograph apparatus comprising a measuring cell and a reference cell constructed in such manner that exterior pressure variations cause equal pressure variations in both cells. The measuring cell is transparent. The quantity to be measured is the pressure difference between the two cells. A mechanism is provided for adjusting the ratio of the volume to the compliance of the reference cell, so that this ratio may be made exactly equal to the corresponding ratio for the measuring cell. The adjustment may be performed by adjusting the force exerted on a membrane, which has been inserted in a wall of the reference cell.