THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



AUGUST 1890. 



XII. Some Experiments on the Velocity of Transmission of 

 Electric Disturbances, and their Application to the Theory of 

 the Striated Discharge through Gases. By J. J. Thomson, 

 M.A.j F.R.S., Cavendish Professor of Experimental Physics, 

 Cambridge* '. 



THE velocity of transmission of an electric impulse along 

 a wire is, according to Maxwell's theory, equal to the 

 velocity with which light passes through the dielectric sur- 

 rounding the wire. The velocity is thus determined by the 

 surrounding dielectric, in which the energy resides, and the 

 function of the wire seems merely to be that of guiding the 

 discharge, which travels at a rate fixed by the dielectric. 



The equality between the velocity of light through air and 

 the rate of transmission of an electric impulse along a wire 

 surrounded by air has been roughly verified by observiug the 

 lengths along a wire of electric waves generated by an electric 

 vibrator : it was found that the wave-length divided by the 

 calculated time of vibration was very nearly equal to the 

 velocity of light. In order to test Maxwell's theory more 

 fully, and to illustrate the influence of the dielectric and the 

 subordinate part played by the conductor, I have made some 

 experiments to compare the velocity of transmission of electric 

 impulses along wires surrounded by different dielectrics. 



If an electric vibrator AC attached to an induction-coil has 

 connected with it a wire AB which bifurcates at B into two 

 circuits, BL, BM, and if the ends L, M are connected to a 

 spark-micrometer the sparking-distance of which can be ad- 

 justed by means of a screw, the sparking-distance will depend 

 * Communicated by the Author. 



Phil. Mag. S. 5. Vol. 30. No.' 183. August 1890. K 



