PROCEEDINGS 



OF THE 



Cambridge lIjHosopbical j&crrictg. 



Monday, 27 January 1896. 

 Professor G. D. Liveixg, Vice-President, in the Chair. 

 The following communications were made to the Society : 



(1) Longitudinal Electric Waves, and Rontgens X Bays. By 

 J. J. Thomson, M.A., F.R.S. 



In the theory of Electric Waves given by Maxwell the only 

 currents considered are (1) displacement or dielectric currents, and 

 (2) conduction currents ; the first of these is proportional to the 

 differential coefficient of the electric intensity with respect to the 

 time, while the second is directly proportional to the electric 

 intensity itself. When we confine our attention to these currents 

 Maxwell's investigation shows that both the electric intensity and 

 the magnetic force are in the wave front. There is thus no 

 longitudinal wave, and X, Y, Z the components of the electric 

 intensity satisfy the condition 



d_{dX dY dZ\ = . 

 dt \dx dy dz\ ' 



this, which expresses that the volume density of the electrification 

 remains constant, is the condition that there should be no longi- 

 tudinal wave. Now though the dielectric and conduction currents 

 leave the volume density of the electrification unaltered, there is 

 a third class of currents, not considered by Maxwell, which may 

 alter the volume density of the electrification ; these are convec- 

 tion currents, i.e. currents due to the motion of electrified bodies 

 through the ether or of the ether past electrified bodies. When 

 these currents exist the condition for the vanishing of the longi- 

 tudinal wave is not satisfied, and in fact in this case we may 

 evidently have longitudinal waves. Thus we know that in a 

 vacuum tube the electrification has a finite value in certain parts 



VOL. IX. PT. II. 4 



