12 



NATURE 



{Nov. 3, i«87 



By thus noticing that the connexion between matter 

 and electricity, known as resistance and defined by 

 Ohm's law, is competent to produce contact electro- 

 motive forces, we may perceive how it comes to pass 

 that in good conductors such forces are so weak, while in 

 insulators they are so strong. Electricity slips through 

 the fingers of a metal as it were, and the driving force it 

 can exert is very feeble ; while an insulator gets a good grip 

 and thrusts it along with violence. 



The metals differ in their gripping power, and, roughly 

 speaking, the best conductor makes the worst thermo- 

 electric substance. A bad conductor, like antimony, or, 

 still better, galena, or selenium, or tellurium, makes a far 

 more effective thermo-electric element than a well-con- 

 ducting metal. Not that specific resistance is all that 

 has to be considered in the matter ; there is also a specific 

 relation between each metal and the two kinds of elec- 

 tricity. Thus, iron is a metal whose atoms have a better 

 grip of positive than of negative electricity, and so a 

 positive current gets propelled in iron from hot to cold. 

 Copper, on the other hand, acts similarly on negative 

 electricity, and it is a negative current which is driven 

 from hot to cold in copper. And all the metals can be 

 classed with one or other of these two, except perhaps 

 lead, which appears to grip both equally, and so to exert 

 no diiferential effect upon either. 



Passage of Electricity through a Gas. 



There remains to be said something about the way in 

 which electricity can be conveyed by gases. 



The first thing to notice is that there is no true con- 

 duction through either gases or vapours ; in other words, 

 a substance in this condition seems to behave as a perfect 

 insulator — perhaps the only perfect insulator there is. 

 Not even mercury vapour is found to conduct in the least. 

 This shows that mere bombardment of molecules, such 

 as is known to go on in gases, is not sufficient either to 

 remove or to impart any electric charge. 



The commonest way in which electricity makes its way 

 through a gas, setting aside the mere mechanical con- 

 veyance by solid carrier, is that of disruptive discharge. 

 Let us try and look into the manner of this a little more 

 closely, if possible. 



First of all, since locomotion is possible to the mole- 

 cules of a gas the same as of any other fluid, it is natural 

 to ask why electrolysis does not go on as in a liquid. 

 Now, for electrolysis in a liquid two conditions seemed 

 necessary : first, that the atoms or radicles in a molecule 

 should be oppositely charged with electricity ; second, 

 that they should be in such a condition (whether by dis- 

 sociation or otherwise) that interchanges of atoms from 

 molecule to molecule, or, in some other way, a procession 

 of atoms, could be directed in a given direction by a very 

 feeble or infinitesimal force. 



Since a gas does not act as an electrolyte, one of these 

 conditions, or perhaps both, must fail. Either the atoms 

 of a gas-molecule are not charged, which is a plausible 

 hypothesis for elementary gases, or else the atoms belong- 

 ing to a gas-molecule remain individually belonging to it, 

 and are not readily passed on from one to another. 



When one says that a gas does not act as a common 

 electrolyte, the experimental grounds of the statement 

 are that a finite electrostatic stress certainly is possible 

 in its interior — a stress of very considerable amount ; 

 and when this stress does overstep the mark and cause 

 the electrode to yield, the yielding is evidently not 

 a quiet and steady glide or procession, but a violent 

 breaking down and collapse, due to insufficient tenacity 

 of something. One may therefore picture the molecules 

 of a gas, between two opposite electrodes or discharge 

 terminals maintained at some great difference of poten- 

 tial, as arranged in a set of parallel chains from one to 

 the other, and strained nearly up to the verge of being 

 torn asunder. In making this picture one need not sup- 



pose any fixture of individual molecules : there may be a 

 wind blowing between the plates ; but all molecules as 

 they come into the field must experience the stress, and 

 be relieved as they pass out. 



If the applied slope of potential overstep a certain 

 limit, fixed by observation at something like 33,000 volts 

 per linear centimetre for common air, the molecules give 

 way, the atoms with their charges rush across to the 

 plates, and discharge has occurred. The number of 

 atoms thus torn free and made able to convey a charge 

 by locomotion is so great that there has never been found 

 any difficulty in conveying any amount of electricity by 

 their means. In other words, during discharge the gas 

 becomes a conductor, and, being a conductor by reason 

 of locomotion of atoms, it may be called an electrolytic 

 conductor. 



But whether the charge then possessed by each carrier 

 atom intrinsically belonged to it all the time, or whether 

 it was conferred upon the components of the molecules 

 during the strain and the disruption, is a point not yet 

 decided. 



What is called " the dielectric strength " of a gas — that 

 is, the strain it can bear without suffering disruption and 

 becoming for the instant a conductor — depends partly on 

 the nature of the gas, and very largely on its pressure. 

 Roughly, one may say that a gas at high pressure is very 

 strong, a gas, at low pressure very weak. An ordinary 

 electrolyte might be called a dielectric of zero strength. 



One reason why pressure affects the dielectric tenacity 

 of a gas readily occurs to one : it is certainly not the only 

 one, but it can hardly help being at least partially a vera 

 causa; and that is, the fact that in a rare gas there are 

 fewer molecules between the plates to share the strain 

 between them. 



Thus if 40,000 volts per centimetre break down ordinary 

 air, 40 volts per centimetre ought to be enough to effect 

 discharge through air at a pressure of about | millimetre 

 of mercury ; and at a pressure of 50 atmospheres 2,000,000 

 volts per centimetre should be needed.' 



A Current regarded as a Moving Charge. 



To review the ground we have covered so far. We first 

 tried to get some conception of the nature of electrostatic 

 charge, and the function of a dielectric medium in static 

 electricity. We next proceeded to see how far the phe- 

 nomena of current electricity could be explained by refer- 

 ence to electrostatics. For a current, being merely 

 electricity in locomotion, need consist of nothing but a 

 charged body borne rapidly along. 



Charge a sphere with either positive or negative elec- 

 tricity, and throw it in some direction : this constitutes 

 a positive or a negative current in that direction. There 

 is nothing necessarily more occult than that. And a 

 continuous current between two bodies may be kept up 

 by having a lot of pith balls, or dust particles, oscillating 

 from one to the other, and so carrying positive electricity 

 one way, and negative the other way. But such carriers, 

 as they pass each other with their opposite charges, 

 would be very apt to cling together and combine. They 

 might be torn asunder again electrically, or they might 

 be knocked asunder by collision with others. Unless 

 they were one or other, the current would shortly have to 

 cease, and nothing but a polarized medium would result. 



Instead of pith balls, picture charged atoms as so act- 

 ing, and we have a rough image of what is going on in 

 an electrolyte on the one hand, and a dielectric on 

 the other. The behaviour of metals and solid con- 

 ductors is more obscure. Locomotive carriage is not to 

 b2 thought of in them ; but, inasmuch as no new pheno- 

 menon appears in their case, it is natural to try and 



' It is true that tension per unit area, or energy per unit volume, is pro- 

 portional to the square of the pofential-slope, and I attach no special import- 

 ance to the simple proportion assumed in the text. There is a great deal 

 more to he sa'd on these subjects, but this is scarcely th^ prjper place to 

 say it. 



