258 Lord Rayleigh on the Vibrations of 



The current produced by an electromotive force in a circuit 

 composed entirely of copper wires joined together by means of 

 binding-screws may, under certain circumstances, be different from 

 the current produced by the same electromotive force acting in the 

 opposite direction. 



I have called this phenomenon " unilateral conductivity/" and 

 I have tried to bring it into connexion with known facts. The 

 most plausible explanation seemed to me to be, that a thin layer 

 of air may sometimes intervene between the two wires which 

 are screwed together. This explanation has been confirmed by 

 some experiments. Other experiments have shown that the ex- 

 planation is insufficient. I do not think that the evidence is 

 sufficiently strong to abandon altogether an explanation which 

 seems to agree so well with the most characteristic features of 

 the phenomenon. Secondary causes may intervene which pre- 

 vent the phenomenon from being formed. I suggest the dif- 

 fusion of the gases into the wires as such a secondary pheno- 

 menon. Effects which are so unstable, however, are never 

 explained by a simple set of experiments. They will only be 

 satisfactorily explained by a number of observations from dif- 

 ferent experimenters. It is, I hope, a sufficient justification for 

 the publication of the above experiments if they draw the atten- 

 tion of physicists to a class of phenomena which sometimes may 

 seriously interfere with their measurements. 



XXXVII. On the Vibrations of Approximately Simple Systems. 

 By Lord Rayleigh, M.A., F.R.S.* 



IN a paper with the above title, published in the Philosophical 

 Magazine for November 1873, I drew attention to the fact 

 that when the natural vibrations of a system are thoroughly 

 known, the effect of a small variation in the system in changing 

 the types and periods of vibration may be readily calculated by 

 a general method. In particular I proved that the altered pe- 

 riods may be found from the new values of the potential and 

 kinetic energies on the hypothesis that the types are unchanged, 

 subject to an error of the second order only. The present note 

 shows how a further approximation may be made, and how a 

 similar method may be applied to a system subject to small dis- 

 sipative forces. 



If <f> v (J)q, &c. be the normal coordinates of the original system, 

 the expressions for the kinetic and potential energies are 



* Communicated by the Author. 



:.*::} • /• • « 



