126 THE CIRCULATION OF THE BLOOD 



Spring Manometer Tracings 



The cause of this inaccuracy depends on the inertia of the mercury, 

 an inertia which is so great that the sudden changes of pressure produced 

 by each heartbeat are not able to overcome it, whereas the much less 

 significant but more prolonged pressure changes produced by each respi- 

 ration develop their full effect on the mercury. These facts led investi- 

 gators to seek for instruments in which the inertia error is eliminated, 

 with the result that they invented what are known as spring manometers. 



Fig. 23. Hiirthle's spring manometer. 



Many forms of this instrument have been devised, but for our pur- 

 pose it is necessary to describe the principle of only the simplest and 

 most efficient the Hiirthle manometer. As shown in Fig. 23, it consists 

 of a variety of tambour, which differs from the ordinary tambour in two 

 important particulars: (1) the chamber is made as small as possible, and 

 (2) it is covered not with an elastic membrane but with one of leather or of 

 thin fluted metal. These two precautions are taken in order to avoid spuri- 

 ous waves set up on account of elastic recoil. Such errors are further 

 reduced by filling the tubing and chamber of the tambour with a fluid 

 so as to eliminate the elastic recoil of air. 



Fig. 24. Arterial pressure recorded by a spring manometer. The effect of weak excitation of 

 the vagus is seen during the period marked by the signal m. (From Dubois.) 



Before the tracing taken with the spring manometer can be em- 

 ployed for quantitative measurements, it must obviously be graduated 

 according to some scale. This is accomplished immediately before or 

 after the experiment by connecting the manometer through a T-piece 

 with a pressure bottle, which can be raised or lowered to a specified 

 height, and with a mercury manometer. The displacement of the writing 

 point of the spring manometer corresponding to each 10 mm. Hg of 

 pressure is then written on the tracing. 



