228 PHYSICAL SCIENCE 



correspond with the new position of the electron 

 at rest, while outside it, in regions as yet unaffected 

 by the change in velocity, the tube ^Q is still 

 moving forward with the original speed of the 

 electron. In the pulse itself, then, the electric 

 tube/^ is bent more or less at right angles to the 

 direction of propagation of the pulse, which spreads 

 out from the electron as centre. When tubes 

 move, a magnetic force is produced at right angles 

 both to their length and to their direction of 

 motion ; and thus, in the thickness of the pulse, 

 a magnetic force exists, also at right angles to the 

 direction of propagation of the pulse, that is, in 

 the plane of the advancing wave-front, and, in 

 that plane, at right angles to the direction of the 

 electric force. The pulse is thus an electro- 

 magnetic disturbance. 



Now, if, instead of imagining the moving 

 electron suddenly brought to rest, we suppose 

 that it is reversed in its path, and that this 

 reversal occurs periodically, so that the electron 

 performs simple harmonic vibrations, we get, 

 instead of a single thin pulse, a series of less 

 abrupt but regularly recurring alternations propa- 

 gated out from the corpuscle as centre. Each 

 Faraday's tube is set into oscillation at its inner 

 end, and transverse waves travel outwards along 

 it, just as waves travel along a stretched cord, 

 when one end is oscillated periodically by the 

 hand. The distribution of electric and magnetic 

 force in the advancing wave-front is exactly the 

 same as in the case of the sudden pulse already 

 studied : we get, in fact, a series of regular 

 aethereal waves, in which there are electric and 

 magnetic forces, both in the plane of the wave- 



