Nov. 3, 1887] 



NA TURE 



n 



point of its course, but it is propelled at the critical 

 points where the special friction occurs, and this comes 

 to sufficiently the same thing. 



We learn, therefore, that stream-lines like Fig. 15 prove 

 one of three things, not one of two ; and the three things 

 are : (i) that the fluid has no friction ; or (2) that it has 

 no inertia ; or (3) that it is propelled at every point of its 

 course. 



If any one of these is true of electricity, there is no 

 need to assume either of the others in order to explain 

 the actual manner of its flow. Now we have just seen 

 that, according to Prof. Poynting's interpretation of 

 Maxwell's theory, the third of the above is true — elec- 

 tricity is propelled at every point of its course ; conse- 

 quently, as said in Part I. (p. 533), the question of its 

 inertia so far remains completely open. 



Voltaic Battery. 



Leaving this singular mode of regarding the subject 

 for the present, to return to it perhaps after Part III., let 

 us proceed to ask how it comes about that a common 

 battery or a thermopile is able to produce a current. 



If we allow ourselves to assume the existence of an 

 unexplained chemical attraction between the atoms of 

 ■different substances, an explanation of the action of an 

 ordinary battery cell is easy. You have first the liquid 

 containing, let us say, hydrogen and oxygen atoms, free or 

 potentially free — that is, either actually dissociated or so 

 frequently interchanging at random from molecule to 

 molecule that the direction of their motion may be 

 guided by a feeble directive force. Each of these atoms 

 in the free state possesses a charge of electricity — the 

 hydrogen all a certain amount of positive electricity, the 

 oxygen twice that amount of negative. Into this liquid 

 you then plunge a couple of metals which attract these 

 atoms differently : for instance, zinc and copper, which 

 both attract oxygen, but zinc more than copper ; or, 

 better, zinc and platinum, the latter of which hardly 

 attracts it at all ; or, better still, zinc and peroxide of 

 lead, one of which attracts oxygen, the other hydrogen. 



Immediately, the free oxygen atoms begin moving up 

 to the zinc, the free hydrogen atoms to the other plate. 



When one speaks of the plates attracting the atoms, it 

 is not necessary to think of their exerting a force on all 

 those in the liquid, distant and near : all that is necessary 

 is to assume a force acting on those which come within 

 what is called "molecular range " of its surface — a dist- 

 ance extremely minute, and believed to be about the ten- 

 millionth part of a millimetre. If the zinc plate removes 

 and combines with all the oxygen atoms which come 

 within this range, they will be speedily replaced by others 

 from the next more distant layer by diffusion, and these 

 again by others, and so on. And thus there will be a 

 gradual procession of oxygen atoms all through the 

 liquid towards the zinc, the rate of the procession being 

 regulated by the force acting, and by the rate of diffusion 

 possible in the particular liquid used. All the atoms 

 which reach the zinc neutralize a certain portion of its 

 electricity by means of the positive charge they carry, 

 and thus very soon it would become positively electrified 

 enough to neutralize its attractive power on the similarly 

 charged oxygen atoms, and everything would stop. But if 

 a channel for the escape of its electricity be provided by 

 leading a wire from it to the copper plate, the circuit is 

 completed, the electricity streams back by the wire, and 

 the procession goes steadily on. The electricity thus 

 imparted to the copper, or platinum, neutralizes any 

 repulsion it exerted on the negatively charged hydrogen 

 atoms, and makes them in a similar way begin a pro- 

 cession towards it, deliver up their charges to it, combine 

 with each other, and escape as gas. 



Without going into all the niceties possible, this 

 mode of thinking of the matter at least calls attention to 

 some of the more salient features of a battery. 



If, instead of two different plates, plates of the same 

 metal be immersed, they will need to be oppositely 

 electrified by some means before they are able to cause 

 the two opposite processions, and so maintain a current 

 in the liquid. This plainly corresponds to a voltameter. 



Taking advantage of the known fact that the atoms 

 are charged, Helmholtz avoids the necessity for postulat- 

 ing any chemical (non-electrical) force between zinc and 

 oxygen, by imagining that all substances have a specific 

 attraction for electricity itself, and that zinc exceeds 

 copper and the other common metals in this respect. 



He would thus think of the zinc attracting, not the 

 oxygen itself, but its electric charge ; and so would liken a 

 battery cell still more completely to a voltameter. The 

 polarization or opposition force acting at the hydrogen- 

 evolving plate he would account for by the attraction of 

 hydrogen for negative electricity, and the consequent 

 repugnance of the hydrogen atoms to part with their 

 charges. 



Thermo-electric Pile. 



A thermopile may be thought of in the following way, 

 but in trying to understand the nature of these actions at 

 present one must admit that some speculation and vague- 

 ness exist. 



We have seen that when electricity is propelled through 

 or among the molecules of a metal it experiences a certain 

 resistance or opposition force which is exactly propor- 

 tional to the speed of its motion. In other words, there 

 is a connexion between matter and electricity in many 

 respects analogous to fluid friction but varying accurately 

 as the first power of the relative velocity. Hence, if an 

 atom of matter be vibrating about a fixed point, it will 

 tend to drive electricity to and fro with it ; but if it be 

 only one of a multitude, all quivering in different phases, 

 they will none of them achieve any propulsion. This 

 may be considered the state of an ordinary warm solid. 

 But if from any cause a set of atoms could be made to 

 move faster in one direction than in the reverse direction 

 — to move forwards quickly and backwards slowly — then 

 such an unsymmetrically- moving set will exert a pro- 

 pulsive tendency and tend to drive a current of electricity 

 forwards, simply because the force exerted is proportional 

 to the velocity, and so is greater on the forward journey 

 than on the return. 



Wherever conduction of heat is going on along a sub- 

 stance the atoms are in this condition. They are driven 

 forward infinitesimally quicker, by the more rapidly moving 

 atoms at the hot end, than they are driven back by the 

 less rapidly moving atoms in front. And hence such a 

 slope of temperature exerts a propulsive tendency : there 

 is an electromotive force in a substance unequally heated. 

 This fact was discovered theoretically and verified 

 experimentally by Sir William Thomson. 



But not only is there such a force at a junction of a 

 hot and cold substance, there is also a force at the junc- 

 tion of two substances of different kinds, even though the 

 temperature be uniform. It is not quite so easy to explain 

 how it now comes about that the atoms at this kind of 

 junction are moving faster one way than the other ; 

 nevertheless, such a thing is not unlikely, considering the 

 state of constraint and accommodation which must 

 necessarily exist at the boundary surface of two different 

 media. However it be caused, there is certainly an 

 E.M.F. at such a junction. 



Thus, then, in a simple circuit of two metals, with their 

 junctions at different temperatures, there are altogether 

 four electromotive forces — one in each metal, from hot to 

 cold or vice versa, and one at each junction ; and the 

 current which flows round such a circuit is propelled by 

 the resultant of these four. 



But the contact force at a junction is by no means con- 

 fined to metals. It occurs between insulators also, and 

 it is to it that the striking effects produced by all fric- 

 tional electric machines are due. 



