THE PULSE WAVE 



163 



at the two points, and by a measurement of this time (t sees.) and of 

 the distance between the two points (I cms.) the velocity of the wave 



is at once given in cms. per sec. by the quotient - . To determine the 



t 



wave length we must divide its velocity by the total time taken for 



the wave to pass any one point. 



Note that in the two tracings the amplitude of the tracing recorded 

 by the lever nearer the syringe is greater than that of the second 

 tracing, which means that the pressure at the second point does not 

 reach so high a value as at the first, some of the energy remaining 

 stored up in the preceding portions of the tube. 



In the later portions of the two recorded waves secondary waves 

 are seen, which are due to the reflection of the primary wave from 



Fm. 



125. — Apparatus foe Studying the Passage of a Pulse Wave along 

 an Elastic Tube. 



the end of the tube, where it impinges on the peripheral resistance. 

 Note that these waves occur at an earlier stage in the second tracing 

 than in the first. In addition to these, other waves are sometimes 

 seen in the early part of the descending portions of the two tracings, 

 which are due to the elastic vibrations of the tube. 



Alterations in the characters of the wave, according to the initial 

 pressure of the fluid in the tube, should also be examined. As we 

 wish to study only a single wave, our object can be best gained by 

 compressing the tube beyond the peripheral resistance, so that no 

 fluid can escape. After each compression of the syringe the pressure 

 of the fluid in the tube then remains permanently raised, and we may 

 thus record a series of waves at gradually increasing pressures. It 

 is found that as the pressure is raised the velocity of the pulse wave 



m 2 



