Oct. 13, 1887] 



NATURE 



559 



MODERN VIEWS OF ELECTRICITY} 



Part I. 



II. 



FIRST you have an inextensible endless cord circulating 

 over pulleys ; this is to represent electricity flowing in 

 a closed circuit. Electromotive forces are forces capable of 

 moving the cord, and you may consider them applied either 

 by a winch, or by a weight on the hook w. A battery cell 

 corresponds to a small weight ; an electric machine to a 

 slow but powerful winch. Clamping the cord with the 

 screw s corresponds to making the resistance of the 

 circuit infinite. Instead of the cord, clamp, and driving 



Fig. 5. — Mechanical analogy of a metallic circuit. 



pulley, one might consider an endless pipe full of liquid 

 with a stop-cock and a pump on it, but for many purposes 

 the cord is sufficient and more simple. In Fig. 5, the only 

 resistance to the motion is friction, and there is no ten- 

 dency to spring back. Fixed beads are shown on the 

 cord to typify atoms of matter, and they may be more or 

 less rough to represent different specific resistances. If 

 the cord be moved, heat is the only result. 



Now pass to Fig. 6. Here the cord is the same as before 



Fig. 6. — Mechanical analogy of a circuit partly dielectric : for instance, of 

 a charged condenser. A is its positive coat, B its negative. 



but the beads are firmly attached to it, so that if it moves 

 they must move with it. They represent, therefore, the 

 particles of an insulating substance. Nevertheless, their 



' Expansion of a lecture delivered by Dr. Oliver Lodge, partly at the 

 London Institution on January i, 1885, and partly at the Midland Institute, 

 Birmingham, November 15, i8S6, but not hitherto published. Continued 

 from p. 536. 



supports are not rigid — they do not prevent the cord 

 moving at all ; they allow what is called electric " dis- 

 placentent^^ not conduction ; they can be displaced a 

 little from their natural position, but they spring back 

 again when the disturbing E. M.F. is removed. The beads 

 in this figure are supposed to be supported by elastic 

 threads : if the cord were replaced by a closed pipe full 

 of water they would be replaced by clastic partitions. 



Apply a given E.M.F. to this cord and a definite dis- 

 placement is produced. One side gets more cord than 

 usual — it is positively charged ; the other gets less — it is 

 negatively charged. If the applied E.M.P\ e.xceeds a 

 certain limit the strain is too great. The elastics break, 

 and you have disruptive discharge with a spark. But even 

 when the strain is only moderate some of the supports 

 may yield viscously, or be imperfectly elastic and permit a 

 gradual extra displacement of the cord, known to tele- 

 graphists as " soaking in." 



When discharge is now allowed, it will not at once be 

 complete ; a large portion of the displacement will be at 

 once recovered, but the rest will gradually " soak out " 

 and cause residual discharges. 



lUL^ 



Fig 7 — Stages in the discharge of a stratified condenser ; showing one v.-ay 

 in which the pheno.nena of " residual charge," "internal charge," and 

 " soaking out " are produced. 



If the dielectric is at all stratified in structure, so that 

 some of the beads allow cord to slip through them — or yield 

 more than others — then this residual charge effect will 

 become very prominent. 



Fig. 7 illustrates the various stages of a stratified di- 

 electric, with layers of imperfect insulatmg power. I. re- 

 presents a recent charge, of E.M.F. 24. II. represents 

 the same after lapse of time, reduced to 17 by partial 

 internal leakage ; and shows internal charge. The circuit 

 itself is supposed to have been perfectly insulating all the 

 time. III. shows the first discharge ; and II 1 1, shows the 

 state attained after again waiting, viz. a residual charge 

 with an E.M.F. 3 in the old direction. 



Return, however, to the simple discharge, and see how 

 it occurs. Will it take place as a simple sliding back of 

 the beads to their old position ? Yes, if the resistance of 

 the circuit is great, but not otherwise. If the cord is fairly 

 free the beads will fly past their mean position, over- 

 shooting their mark, then rebound, and so, after many 

 quick oscillations, will finally settle down in their 



