hertz's experiments. 227 



a bit of the rope; a link lias an individuality that identities it, while a 

 Int of tltei'o])e is so indefinite that it is not so easy to keep in mind any 

 ])aiti<'ular bit. 



Consider now what these waves are. what sort of motion oriiiinates 

 them. When the spiinj^ first starts, the near ]»aits of the chain 

 mo\es first. What happens to any link? One end of it moves down 

 before the other. What sort of motion then has the link? It must 

 be rotatinji'. Thus it is that chanye in the disphicement is j>enerally 

 accompanied by rotation of the links. Thus it is that change in 

 the electric strain is accompanied by nuignetic strain. The analogy 

 goes farther than this. Ea(;h wave thrown oft' may be described as a 

 wave of displaced — or as a wave of rotating — links, and the most dis- 

 placed are at any time the most rapidly rotating links. Just m the 

 same way, what have hitherto been called waves of electric force may 

 also be looked upon as waves of magnetic force. Because there are two 

 as])ects in which the motion of the chain may be viewed does not 

 diminish from the essential unity of character of the wave motion in its 

 waves; and similarly the fact that these Hertzian waves have an elec- 

 tric and a magnetic as])ect does not diminish from the essential unity 

 of character of the wave motion in them. At the same time the two 

 elements, the displacement of a link and the rotation of a liidv, are 

 (juite distinct things; either might exist without the other; it is only 

 in wave propagation that they essentially co-exist. In the same way 

 electric strain and magnetic strain are quite different things; thongh 

 in wave motion, and indeed whenever energy is transmitted from one 

 place to another by means of the icther, they essentially co-exist. 



