i 3 o6 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



of these types is that they are placed radially so that 

 all fluid particles moving across a radius between the 

 axis and the wall will participate in causing the 

 needle's deflection. 



In the ■'pendulum'" type of figure i fd to/, however, 

 the device is represented either by an elastically sus- 

 pended coaxial cylinder (fig. i 7/) or by a needle which 

 carries at its tip a body with a resistance to the flow- 

 that is very great as compared to that of the needle 

 (fig. \-jd and e). In these cases, the deflections are 

 caused mainly by the axial flow. This renders greater 

 sensitivity than a simple needle, but interferes with 

 the flow to a higher degree and increases the errors 

 which result from changes of the velocity profile. 



The first model of a hydrometric pendulum was 

 employed by Castelli [1 577-1 644, quoted from (96)]. 

 It consisted of a sphere which was suspended on a 

 thread and submerged in the streaming water. The 

 angular deflection of the thread from the vertical was 

 used as a measure of the flow velocity. In this author's 

 honor, M tiller (96) proposed that the principle of all 

 similar flowmeters be called the "Castelli principle." 

 On an analogous theoretical basis, Michelotti (1710- 

 1777) built a hydraulic balance (96). 



Yierordt [1858, quoted from (38)] was the first to 

 apply the hydrometric pendulum to the measurement 

 of blood velocity, but his instrument was far from 

 perfect. 



The hemodromograph of Chauveau and Lortet 

 [i860 and 1867, quoted from (38)] was the first model 

 of a hvdrometric pendulum which allowed continuous 

 recording of the blood velocity. The design was 

 similar to that of figure lje. A rubber diaphragm 

 represented the fulcrum beyond which the lever of 

 the pendulum was prolonged so as to act on an air- 

 transmission system. Noteworthy progress was 

 achieved by this device although the natural fre- 

 quency was not high enough and blood pressure varia- 

 tions changed the position of the rubber diaphragm 

 which held the pendulum. 



Frank (40) presented some basic theoretical prin- 

 ciples of the hydrometric pendulum and of the 

 mechanical conditions necessary for adequate fre- 

 quency response. He also stated that submerging the 

 pendulum into fluid markedly increased the damping 

 of free vibrations while only slightly lowering the 

 natural frequency- From this he concluded that the 

 mass of fluid adhering to the pendulum must be rela- 

 tively small. He built an elastically suspended pendu- 

 lum for optical registration (40). 



The first attempt at transmitting the pendulum 

 deflections electrically was made by de Burgh Daly 



fig. 18. Bristle flowmeter of 

 Holzlohner and Bergmann. GL, 

 glass cannula; BO, bristle; E, 

 and £2, platinum electrodes; S, 

 vertical tube. [From Bergmann 

 (5)-] 



(21), who installed, in the vertical limb of a T-can- 

 nula, a condenser composed of two copper foils, one 

 being attached to the pendulum rod and the other 

 being placed on the outside surface of the side tube. 

 The variations of capacity of this condenser were 

 caused by the pendulum deflections and detected by 

 high frequency and rectifier circuits. 



Another type of electric transmission was applied 

 by Holzlohner (61), who used the streaming blood 

 itself as an electrical conductor. In his device, three 

 electrodes are placed in the flow cannula (fig. 18) so 

 that they form two limbs of a Wheatstone bridge. The 

 bristle, belonging to the type of figure 1 ja, represents 

 the middle electrode and consists of a thin platinum 

 wire covered by a fine glass coating. At the bristle tip, 

 the wire is bare and in direct contact with the blood. 

 Two other platinum electrodes are placed in the tube 

 wall upstream and downstream at equal distances 

 from the bristle tip. When the bristle is bent by the 

 blood stream, the ratio of the resistances on both 

 sides of the bristle tip is changed. The electric equip- 

 ment contains the resistors completing the bridge 

 circuit, a 20-kc per sec oscillator, an a-c amplifier, and 

 a rectifier. Holzlohner called his instrument "•Strom- 

 (borste." By translating this name into English (Borste 

 = bristle), Brecher (12) introduced the expression 

 "bristle flowmeter" into the literature, where it is now 

 ' commonly used in connection with all types shown in 

 figure 17a to c. Holzlohner's model was modified by 

 Bergmann (5) and employed for recording flow in the 



