Oct. 6, 1887] 



NATURE 



535 



and let, say, a suspended gilt pith ball hang between 

 them. Being a conductor there is no strain inside it, 

 and so it acts partially as a bridge, and several of the 

 lines pass through it — or, rather, they end at one side of it 

 and begin at the other : thus it has opposite charges on 

 its two faces — it is under induction (Fig. 3). Let it now 

 be moved so as to touch one of the disks, the lines 

 between it and the disk on that side have shut up, and it 

 remains with those only which go to the other disk. In 

 other words, it has received some lines from the touched 

 disk. These will pull it over to the far disk and there shut 



Fig. 2. — Rough diagram of the state of the medium near two oppositely 

 charged disks when separated. 



themselves up. From that disk it receives more, and 

 travels with their ends back to the first disk, and so on 

 (Fig- 4), perpetually receiving lines and shutting them 

 up until they are all gone and the disks are discharged. 



This mode of stating the facts involves no hypothesis 

 whatever— it is the simple truth. But the " lines of force" 

 have no more and no less existence than have " rays of 

 light." Both are convenient modes of expression. 



But so long as we adhere to this mode of expression we 

 cannot form a complete mental picture of the actually 



FlC. 3.— Rough diagram of the medium between two disks disturbed by the 

 presence of an uncliarged metal sphere. The two halves of the sphere 

 are oppositely charged " by induction." 



occurring operations. In optics it is usual to abandon 

 rays at a certain stage and attend to the waves, which we 

 know are of the essence of the phenomenon, though we 

 do not know yet very much about their true nature. 



Similarly in electricity, at a certain point we are led to 

 abandon lines of force and potential theories, and to try 

 to conceive the actual stuff undergoing its strains and 

 motions. It is then we get urged towards ideas similar 

 to those which are useful in treating of the behaviour of 

 an incompressible fluid. 



In an utterly modified sense, we have still a fluid theory 

 of electricity, and a portion of the ideas of the old theories 

 belong to it also. 



Thus Franklin's view that positive charge was excess 

 and negative charge was a deficit in a certain standard 

 quantity of the fluid which all bodies naturally possessed 

 in their neutral state, remains practically true. His view 

 that the fluid was never manufactured, but was taken 

 from one body to give to another, so that one gained 

 what the other lost — no more and no less — remains 

 practically true. Part also — a less part — of the two-fluid 

 theory likewise remains true, in my present opinion ; but 

 this is not a branch of the subject on which I shall enter 

 in the present discourse. It will suffice for the' present to 

 fix our attention on one fluid only. 



You are to think of an electric machine as a pump 

 which, being attached to two bodies respectively, drives 

 some electricity from the one into the other, conferring 

 upon one a positive and upon the other a precisely equal 

 negative charge. One of the two bodies may be the 

 earth, in which case the charge makes little or no differ- 

 ence to it. 



But, as has been objected before, if electricity is like an 

 incompressible and inextensible fluid, how is it possible to 



Fig. 4. — Rough diagram of the medium near two oppositely charged disks 

 between which a metal carrier ball is oscillating, having just touched the 

 right-hand disk. (Discharge by "alternate contact.") 



withdraw any of it from one body and give it to another 1 

 With rigid bodies it is not possible, but with elastic bodies 

 it is easy. 



The act of charging this sphere is therefore analogous 

 to pumping water into this elastic bag, or rather into a 

 cavity in the midst of an elastic medium, whose thick 

 walls, extending in all directions and needing a great 

 pressure to strain them, better represent the true state of 

 the case than does the thin boundary of a bag like this. 



Draw a couple of such cavities and consider fluid 

 pumped from one into the other, and you will see that the 

 charge {i.e. the excess or defect of fluid) resides on the 

 outside. You may also show that when both are similarly 

 charged the medium is so strained that they tend to be 

 forced apart ; whereas when one is distended and the 

 other contracted they tend to approach. 



Further you may consider two cavities side by side, 

 pump fluid into (or out of) one only, and watch the effect 

 on the other. You will thus see the phenomena of 

 induction, the near side of the second cavity becomh'g 



