186 Lord Rayleigh on an Instrument capable of 



sets of forces, 





/< 2 



pa 



The influence of electrical charge in diminishing the sta- 

 bility of a cylinder for transverse disturbances may be readily 

 illustrated by causing a jet of water from an elliptical aper- 

 ture to pass along the axis of an insulated inductor-tube, 

 which is placed in connexion with an electrical machine. The 

 jet is marked with a recurrent pattern, fixed in space, whose 

 wave-length represents the distance travelled by the water in 

 the time of one vibration of type n = 2. When the machine 

 is worked, the pattern is thrust outwards along the jet, indi- 

 cating a prolongation of the time of transverse vibration. 

 The inductor should be placed no further from the nozzle than 

 is necessary to prevent the passage of sparks, and must be 

 short enough to allow the issue of the jet before its resolution 

 into drops. 



The value of T being known (81 C.Gr.S.), we may calcu- 

 late what electrification is necessary to render a small rain- 

 drop of, say, 1 millimetre diameter unstable. The potential, 

 expressed in electrostatic measure, is given by 



V=^= v /(16 ff a T)=20. 



«o 



The electromotive force of a Daniell cell is about "004; so 

 that an electrification of about 5000 cells would cause the 

 division of the drop in question. 



XXI. On an Instrument capable of Measuring the Intensity of 

 Aerial Vibrations. By Lord Rayleigh, F.R.S.* 



THIS instrument arose out of an experiment described 

 in the ' Proceedings of the Cambridge Philosophical 

 Society ' f, Nov. 1880, from which it appeared that a light 

 disk, capable of rotation about a vertical diameter, tends with 

 some decision to set itself at right angles to the direction of 

 alternating aerial currents. In fig. 1, A is a brass tube closed 

 at one end with a glass plate B, behind which is a slit C 

 backed by a lamp. D is a light mirror with attached mag- 

 nets, such as are used for reflecting-galvanometers, and is sus- 

 pended by a silk fibre. The light from the slit is incident 



* Communicated by the Author. 



t See also Proc. Roy. Soc. May 5, 1881, p. 110. 



