The oscillating mercurial column does not give the variations in blood-pressure quite truly, because the oscillations are compounded of these variations and of the oscillations due to the inertia of the mercury itself. In order to obtain the exact form of variation we employ Fick's kymograph (Fig. 86), or Roy's tonometer, in which the apparatus is made very light, and all oscillations due to its own inertia are as far as possible avoided.

Fig. 86.   Fick's kymograph. It consists of a flat metal tube, bent into a nearly circular form, filled with alcohol, and connected with the artery by means of a leaden tube, filled with a solution of sodium carbonate.

Fig. 86. - Fick's kymograph. It consists of a flat metal tube, bent into a nearly circular form, filled with alcohol, and connected with the artery by means of a leaden tube, filled with a solution of sodium carbonate. When the pressure increases within it, the tube straightens, and when the pressure diminishes it bends. These changes are magnified and recorded on a cylinder by a light lever. The vibrations of the lever are lessened by a piston, which works in a tube filled with glycerine.