I 



Oct. 13, 1887] 



NATURE 



561 



Start with the gauge b at zero, and the gauge a at high 

 pressure. Open stop cock A ; some water is squeezed out 

 of inner coating, and the a gauge falls to zero, but the 



ck of the contracting bag on the outer coat pulls down 



Fig. II. — First actually constructed model Leydea jar, with mercury gauges 

 for electrometers ; the whole rigged up with things purchasable at a 

 plumber's, except the pump. 



the gauge b below zero, the descent of the two gauges 

 being nearly equal. 



Next shut A and open B ; a little water flows in from 



Fig. 12. — Latest (orm (jt liydrauUc niudel of a l^cyjcn jar with water gauges, 

 the whole arranged vertically to be more conspicuous. The pump is a 

 force-pump with a communication between top of barrel and tank to get 

 rid of^stray water. 



the tank to still further relieve the strain of the bag, and 

 both gauges rise ; b to zero, a to something just short of 

 its old position. 



Now shut B and open A again : again the two gauges 

 descend. Reverse the taps, and again they both rise ; 

 and so on until the bag has recovered its normal size. 

 This is discharge by alternate contact, and exactly 

 imitates the behaviour of an insulated charged Leyden 

 jar whose inner and outer coats are alternately touched 

 to earth. Its pith balls alternately rise with positive and 

 with negative electricity, indicating potentials above and 

 below zero. 



Figs, 1 1 and 12 are taken from photographs of apparatus 

 I have made to use as just described. The glass globe 

 with the partially distended bag inside it, the pump, the 

 tank, the gauges a and b, the stop-cocks A B c, will be 

 easily recognized. Two extra stop-cocks. A' and B', leading 

 direct to tank, are extra, and are to save having to dis- 

 connect pump and connect A direct, when exhibiting the 

 effect of " discharge by alternate contact." but the tank 

 is not sufficiently tall in Fig. 12 ; I have doubled its 

 height since. The full height of the gauge-tubes is barely 

 shown. 



In any form of apparatus it is essential to fill the 

 whole with water — pipes, globe, everything— before com- 

 mencing to draw any moral from its behaviour. It is 

 rather difficult to get rid of a large bubble of air from 

 the top of the globe of Fig. 11, and though it is not 

 of very much consequence in this place, the stopcock in 

 Fig. 12 is added to make its removal easy. The gauges in 

 Fig. 1 1 may be replaced by others arranged as a lantern- 

 slide, and connected by flexible tubing full of air. 



I have explained thus fully the hydraulic illustration of 

 Leyden jar phenomena, because these constitute the key 

 to a great part of electrostatics. The illustration is not 

 indeed a complete or perfect one by any means, but by 

 combining with it a consideration of the endless cord 

 models, and of what I have endeavoured to explain con- 

 cerning conduction and insulation in general, a distinct 

 step may be gained. 



Think of electrical phenomena as produced by an all- 

 permeating liquid embedded in a jelly ; think of con- 

 ductors as holes and pipes in this jelly, of an electrical 

 machine as a pump, of charge as excess or defect, of 

 attraction as due to strain, of discharge as bursting, of 

 the discharge of a Leyden jar as a springing back or 

 recoil oscillating till its energy has gone. 



By thus thinking you will get a more real grasp of the 

 subject and insight into the actual processes occurring in 

 Nature — unknown though these may still strictly be — than 

 if you employed the old ideas of action at a distance, or 

 contented yourselves with no theory at all on which to 

 link the facts. You will have made a step in the direction 

 of the truth, but I must beg you to understand that it is 

 only a step, that what modifications and additions will 

 have to be made to it before it becomes a complete theory 

 of electricity I am wholly unable to tell you. I am con- 

 vinced they will be many, but I am also convinced that it 

 is unwise to drift along among a host of complicated 

 phenomena without guide other than that afforded by 

 hard and rigid mathematical equations. 



The mathematical theory of potential and the like has 

 insured safe and certain progress, and enables mathe- 

 maticians to dispense for the time being with theories of 

 electricity and with mental imagery. Few, however, are 

 the minds strong enough thus 10 dispense with all but 

 the most formal and severe of mental aid^ : and none, 

 I believe, to whom some mental picture of the actual 

 processes would not be a help if it were safely available. 



Such a representation I have endeavoured partially 

 to lay before you to-night, and I hope, if 1 have succeeded 

 in making myself at all intelligible, that those students of 

 electricity who may be present will find it of some use 

 and service. O. J. Lodge. 



[To be continued.) 



