the Velocity of Electric Waves. 213 



Every time the primary condenser was charged a spark 

 passed at C, causing an oscillatory discharge. A convenient 

 method of forming a mental picture of the oscillation excited 

 in the secondary circuit is the conception of Faraday tubes 

 elaborated by J. J. Thomson, in his i Recent Researches in 

 Electricity and Magnetism.' The oscillations of the primary 

 acted inductively upon the secondary and sent out groups of 

 Faraday tubes which travelled along the secondary circuit, 

 with their ends on the wires, and lying chiefly in the space 

 between them. At the end J they reversed their direction 

 and travelled back along the circuit. The period of oscillation 

 of the primary circuit was altered, until by trial it was found 

 that groups of returning tubes met groups of advancing tubes 

 between the points G and H. As the two sets of moving 

 tubes were oppositely directed, they annulled each other and 

 produced a node. Thus a system of stationary waves was set 

 up with a node at J, another node at G H, and a ventral seg- 

 ment at K L. The method of discovering when the circuits 

 were in tune and of investigating the shape of the waves will 

 be described later. The point to be noticed here is that the 

 vibrations were sufficiently powerful to cause a luminous dis- 

 charge on the surface of the wire where the accumulation of 

 tubes was a maximum, i. e. at K L, while at the nodal points 

 J and GH the wire was entirely dark. Still further, the wave 

 formation could be made apparent to the sense of hearing as 

 well as that of sight ; for placing the ear within a few 

 centimetres of the wire and walking beside it, a distinct 

 crackling sound could be heard at the points K and L, 

 whereas no such sound could be heard at G, J, and H. By 

 placing bits of glass tubing on the wire the sound was much 

 intensified at the points K and L, and the phenomena made 

 more striking. It might be supposed that by decreasing 

 the capacity of the primary condenser, and therefore the 

 period of its oscillation, the secondary circuit could be broken 

 up into a new set of shorter stationary waves, with nodes at 

 J and at points somewhere near K, L, G, and H, and 

 ventral segments between them. This we tried with perfect 

 success, except that it was not possible to cause the light at 

 K and L to actually disappear. There was decidedly less 

 light at these points, however, than on either side of them. 

 The light of course is simply that which always appears 

 around wires carrying very high potential currents, the 

 interesting point being that it appears in some places on the 

 circuit and not in others. The experiment showing how the 

 circuit breaks up in several different ways would form a most 

 beautiful lecture experiment. 



