of Electric Induction and Conduction. 371 



a cord, though well enough fitted to picture a linear electric 

 current or a stream-line, is not well adapted for the represen- 

 tation of the continuous medium at rest. One can, however, 

 partly get over this by supposing that the cords are lying in 

 all directions, and that all are connected together in some way*. 



Let us then picture to ourselves a dielectric medium as an 

 assemblage of particles (or buttons) joined together by elastic 

 strings and threaded by straight inextensible cords which form 

 three mutually connected sets, each set at right angles to the 

 other two. Imagine a set of transverse waves running down the 

 set of cords parallel to the axis of z. The cords in the direction 

 x and y will be shaken backwards and forwards with their but- 

 tons; but no energy will be lost, and the disturbance will pass 

 right through the medium at a rate dependent on the z cords 

 themselves, on the masses of the particles, and on the elasti- 

 city of their joining threads. An insulating medium is then 

 a transparent medium, provided it is sufficiently homogeneous 

 not to scatter the light, and provided it does not contain pig- 

 ments or other foreign absorbent materials. 



But now form an image of a metallic conducting medium : 

 the cords, as before, thread particles in three cardinal direc- 

 tions ; but the particles are now very smooth and are connected 

 to each other, not by elastic strings, but by nearly rigid rods. 

 Oscillations travelling down the axis of z displace, indeed, the 

 x and y cords ; but since these slide through their buttons, no 

 restoring force is thereby called out, and the disturbance 

 does not succeed in penetrating far into the medium before 

 its energy is all lost in friction — converted into heat. A con- 

 ducting body is necessarily an opaque body (see Maxwell, 

 art. 798). 



In a dielectric the connexion between a button and the cord 

 is good, but the connexion between one button and another is 

 lax. Motion of the cord is therefore readily and quickly im- 

 parted to the buttons, or vice versa ; i. e. dielectrics are good 

 absorbers and radiators |. Motion of a button is only slowly 

 transmitted to other buttons ; they are in general bad conduc- 

 tors of heat. In a metal, on the other hand, it is not easy to 

 set the buttons swinging by means of the cord ; but if a button 

 be once set in motion, its motion is rapidly transmitted (by 

 conduction) through the mass. There is here no explanation 



* Maxwell's theory is not responsible for the tendency in the model to 

 make out that an electric current is a current of aether. 



t Cf. Rankine " On the Hypothesis of Molecular Vortices/' Phil. Mag. 

 July 1851, p. 62, supposition 3. The "atmospheres " of his atoms appear 

 to correspond to buttons, while their " nuclei " agree to some extent with 

 the cord. 



2 B 2 



