1886.] 



On the Sympathetic Vibrations of Jets. 



371 



A jet of air usually responds most energetically to some particular 

 tone or set of related tones (Sondhauss). Such a particular tone 

 may "be called the jet fundamental. The practical inconvenience 

 arising from this may be diminished by raising the air-pressure until 

 the jet fundamental is higher than any of the tones to be repro- 

 duced. 



When a flame and an air-jet meet at right angles vibrations im- 

 pressed upon the flame orifice also yield sound. The conditions of 

 pressure, &c, are somewhat different ; but the changes produced at 

 the orifice grow in the same way as those in an air- jet, The best results 

 are obtained when a gentle current of air is directed from a wide 

 tube just below the apex of the blue zone. 



It is difficult at first sight to account for the fact that a vibrating 

 jet gives rise to sound only when it strikes upon some object which 

 divides it into two parts. The following experiments, however, in 

 some sense explain this. The relative normal velocity at different 

 points in the stream may be measured by introducing into its path 

 the open end of a capillary tube which is connected with a water 

 manometer. This velocity diminishes continuously along the axis 

 from the orifice to the breaking point ; and also diminishes continu- 

 ously from any point of the axis outwards towards the circumference. 

 Now a sudden disturbance communicated to the air at the orifice will 

 be found to produce a, fall in velocity along the axis of the jet, but a 

 rise in velocity along its extreme outer portions. It thus appears that 

 the changes along the axis and along the circumference,, produced by 

 a disturbance, are of opposite character. When the jet plays into 

 free air these opposing changes neutralise each other in the main ; but 

 this interference is prevented when the jet strikes upon any object 

 which serves to divide it. 



When a vibrating air-jet plays against a small flame, the best 

 sounds are heard when the stream strikes the flame just below the 

 apex of the blue zone. At the plane of contact an intensely blue 

 flame ring appears, and this ring vibrates visibly when the jet is 

 disturbed. The production of sound from it doubtless depends on 

 changes in the rate of combustion of the gas. This may be proved 

 by inserting into the ring a fine slip of platinum, connected in circuit 

 with a battery and a telephone. When the jet is thrown into vibra- 

 tion the consequent variations in the temperature of the platinum 

 affect its conductivity, and hence a feeble reproduction of the jet 

 vibration may be heard in the telephone. 



To Savart we are mainly indebted for our knowledge of the 

 sympathetic vibrations of liquid jets. This physicist showed that a 

 liquid jet always tends to separate into drops at a distance from the 

 orifice in a regular manner; and that this tendency is so well marked, 

 that when the jet strikes upon any object, such as a stretched mem- 



