364 Mr. 0. J. Lodge on a Mechanical Illustration 



nearly inextensible that impulses are propagated along it with 

 the velocity of light. But if the wire, instead of hanging in 

 free space, is coated with some insulating material and im- 

 bedded in the ground, the propagation of potential goes on 

 comparatively slowly. . There are, in fact, besides the simple 

 circuit straight through the wire and back through the earth, a 

 multitude of lateral circuits, each represented by a separate cord, 

 one for each unit area of the dielectric. When an electromotive 

 force, say, a negative potential or a pull, acts at one end of such a 

 compound circuit, its first effect is to pull out those cords which 

 offer least resistance ; in fact the current is split among the 

 several circuits in the inverse ratio of their respective resist- 

 ances. The direct and longest cord feels a slight instanta- 

 neous pull indeed; but it is only slight until the cords through 

 the dielectric have been pulled and a tension set up all along 

 the line. While this is going on ; the pull on the direct cord is 

 gradually increasing. It would be possible to represent all 

 this by a sufficiently elaborate model ; but the action may be 

 imitated with only a single cord if it be arranged zigzag on 

 pulleys supported by elastic strings, as shown in fig. 4. A 

 sudden pull at one end of such a cord will stretch the elastics 

 of the near pulleys ; the strain so caused will stretch the next, 

 and so on ; and thus the impulse will be transmitted along the 

 cord, getting, however, weaker and weaker as it advances. A 

 still simpler imitation of the effects observed would be obtained 

 by making the direct cord itself elastic ; but such an illustration 

 would have very little analogy with the process really going on. 



Electrolytic Conduction. 



§ 13. A model intended to represent the passage of a cur- 

 rent through an electrolyte is shown in fig. 5 ; but it is not 

 intended to be actually constructed *. In this figure the buttons 

 are supported by rigid rods ; but instead of these being 

 screwed to beams as in fig. 3, they are attached to rings which 

 slide freely on two glass rods a and b. The additional piece 

 Z is a knife-edge mounted on elastic nillars or spiral steel 

 springs, and placed close to the cord, so that its sharp edge is 

 in the path of the buttons if they travel along. The friction 

 between the cord and the buttons is supposed to be practically 

 infinite. 



Now hang a weight W on to its hook at D and note what 

 occurs. The cord moves from A toward B, carrying with it 

 the buttons, which travel bodily forward without bending their 



* The electrolytic model here described is capable of considerable sim- 

 plification and improvement, 



