Vessel 1 is connected to membrane box 2 and also to the atmosphere through capillary tube 3. Depending upon the vertical velocity of the item on which the instrument is mounted, the pressure inside and outside membrane box 2 differs due to the retarded flow of air through the narrow passage in capillary tube 3. This deforms membrane box 2 and its motion is transmitted to shaft 10 to which hand 4 is attached. The hand is turned by means of link 7, connected by turning pairs A and B to membrane box 2 and to lever 8, turning about fixed axis C, and flexible link 9 which is wound aroun drum a. Drum a is rigidly mounted on shaft 10 which turns about fixed axis D. Capillary tube 3 is enclosed in a metal housing connected to the atmosphere through valve 5 which can be closed with handle 6. The instrument is protected against overloads by valve 11 which opens automatically and admits air (in climbing) or releases air (in descending) into or from membrane box 2 through the orifice in nozzle 12. This limits the load on the membrane box.
$4062$EHP,M$