SURFACE-TENSION OF WATER BY THE METHOD OF JET VIBRATION. 299 



Production of the Jet. 



The most important question in the experiments is to produce a jet which, while 

 satisfying the suppositions made in the theoretical development, executes vibrations 

 of a single type. 



This demand, however, cannot be expected to be satisfied by a portion of a jet of 

 liquid which lies at a short distance from the orifice. Apart from possible variations 

 of the value of the surface-tension on account of the very rapid extension of the 

 surface, it will be very difficult to obtain pure harmonic vibrations of the jet at this 

 place, for this requires not only a definite form of the cross-section of the jet at the 

 orifice, but also a definite velocity at every point of this section. While it might be 

 possible to satisfy the first condition by suitable choice of the orifice (see p. 296, (38)), 

 it would, no doubt, be very difficult to satisfy the last ; among other reasons the 

 velocity of the fluid will, for various causes, be greater in the middle of the jet than 

 closer to the surface. It is, therefore, of great importance to produce a jet which is 

 so stable that the vibrations can be examined at a considerable distance from the 

 orifice where the viscosity of the liquid has had time to act. 



A jet issuing from a hole in a thin plate is, however, not very stable, and therefore 

 the jet rather rapidly falls into drops. Tf, however, drawn-out glass-tubes are used 

 as orifice, very long and stable jets can be formed when the tube has a suitable 

 shape. 



In the experiments, jets were exclusively employed the qualities of which repeated 

 themselves twice over the circumference. 



The orifices of the glass-tubes employed were given an elliptic section by specially 

 heating the tubes, before drawing them out, on two opposite sides. Twisting of the 

 glass-tubes would produce a rotation of the jet about its axis, and the planes of 

 vibration would not preserve the same direction at different distances from the orifice ; 

 to avoid such results it was necessary during the heating and drawing-out to have 

 both ends of the tube fastened on slides which could be displaced along a 

 metal-prism. 



When the glass-tubes were drawn out and cut off, they were examined under a 

 microscope, and only those whose orifice had a uniform elliptic section were used. 

 After this the jets, which were formed by the tubes, were examined. The purpose of 

 this examination, which will be mentioned later (p. 307), was to find out if the jet was 

 symmetrical about two perpendicular planes passing through its axis. 



It has been mentioned above that, because of the effect of viscosity, a portion of 

 the jet will execute vibrations, which are in better conformity with those wanted the 

 more removed it is from the orifice. It might here be of interest to illustrate this by 

 an example. 



As such can be employed an experiment carried out with tube I (see the table on 

 p. 310). 



2 Q 2 



