0°- 



NA TURE 



[_Oct. 20, 1887 



the same portion which illuminated the clouds directly overhead 

 at the time of observation. The rainbow suffered no diminution 

 of brightness where it was apparently crossed by the fluted 

 shadows, the latter being far away in comparison with the bow- 

 producing raindrops, which, of course, were in sunshine. 



I regret that I am unable to send a photograph or sketch of the 

 phenomenon, which was a most beautiful one, and must be of 

 rare occurrence. I have never before seen anything similar, nor 

 have I read anywhere a description of a rainbow after sunset. 



Allahabad, India, September 18. S. A. Hill. 



Occurrence of Sterna anglica in Belfast Lougb. 



It may possibly interest some of your ornithological readers 

 to know that towards the end of September a specimen of the 

 gul. -billed tern {Sterna anglica) was shot in Belfast Lough. 

 The bird was placed in the hands of Mr. Darragh, of the 

 Museum of that town, and brought by him to me for determina- 

 tion. On consulting the last edition of " Yarrell," I find that 

 it does not appear to have been previously recorded from Ireland. 



Robert O. Cunningham. 



Queen's College, Belfast, October 8. 



MODERN VIEWS OF ELECTRICITY} 



Part II. 



III. 



"\1/'E have now glanced through electro-static pheno- 

 * *^ mena, and seen that they could be all compre- 

 hended and partially explained by supposing electricity 

 to be a fluid of perfect incompressibility — in other words, 

 a liquid — permeating everywhere and everything ; and 

 by further supposing that in conducting matter this 

 liquid was capable of free locomotion, but that in insu- 

 lators and general space it was as it were entangled in 

 some elastic medium or jelly, to strains in which electro- 

 static actions are due. This medium might be burst, in 

 a disruptive discharge, but easy flow could go on only 

 through channels or holes in it, which therefore were 

 taken to represent conductors ; and it was obvious that 

 all flow must take place in closed circuits. 



To day I want to consider the circumstances of this 

 flow more particularly : to study, in fact, the second divi- 

 sion of our subject (see classification on page 532), viz. 

 Electricity in locomotion. 



I use the term " locomotion " in order to eliminate rota- 

 tion and vibration : it is translation only with which we 

 intend now to concern ourselves. 



Consider the modes in which w<2/^rmaybe made to 

 move from place to place ; there are only two : it may 

 be pumped along pipes, or it may be carried about in 

 jugs. In other words, it may travel through matter, or it 

 may travel Tf///^ matter. Just so it is with //^a/ also : heat 

 can travel in two ways : it can flow through matter, by 

 what is called " conduction," and it can travel with 

 matter, by what is called "convection." There is no 

 other mode of conveyance of heat. You frequently find 

 it stated that there is a third method, viz. "radiation" ; 

 but this is not truly a conveyance of heat at all. Heat 

 generates radiation at one place, and radiation repro- 

 duces heat at another ; but it is radiation which travels, 

 and not heat. Heat only naturally flows from hot bodies 

 to cold, just as water only naturally flows down hill ; but 

 radiation spreads in all directions, without the least atten- 

 tion to where it is going Heat can only flow one way 

 at any given point, but radiation travels all ways at once. 

 If water were dissociated on one planet into its consti- 

 tuent gases, and if these recombined on another planet, it 

 would not be water which travelled from one to the other, 

 neither would the substance obey the laws of motion of 

 water — water would be destroyed in one place, and repro- 



' Continued from p. s'l. 



duced in another ; just so is it with the relation between 

 radiation and heat. 



Heat, then, like water, has but two direct modes of 

 conveyance from place to place. For electricity the same 

 is true. Electricity can travel with matter, or it can travel 

 through matter ; by convection or by conduction, but in 

 no other known way. 



Conduction in Metals, 



Consider, first, conduction. Connect the poles of a 

 voltaic battery to the two ends of a copper wire, and think 

 of what we call " the current." It is a true flow of electricity 

 among the molecules of the wire. If electricity were a 

 fluid, then it would be a transport of that fluid ; if electri- 

 city is nothing material, then a current is no material 

 transfer ; but it is certainly a transfer of electricity, what- 

 ever electricity may be. Permitting ourselves again the 

 analogy of a liquid, we can picture it flowing through, or 

 among, the molecules of the metal. Does it flow through 

 or between them ? Or does it get handed on from one to 

 the next continually ? We do not quite know; but the 

 last supposition is often believed to most nearly represent 

 the probable truth. The flow may be thought of as a 

 perpetual attempt to set up a strain like that in a di- 

 electric, combined with an equally perpetual breaking 

 down of every trace of that strain. If the atoms be con- 

 ceived as little conductors vibrating about and knocking 

 each other, so as to be easily and completely able to pass 

 on any electric charge they may possess, then, through a 

 medium so constituted, electric conduction could go on 

 much as it does go on in a metal. Each atom would 

 receive a charge from those behind it, and hand it on to 

 those in front of it, and thus may electricity get conveyed 

 along the wire. Do not, however, accept this picture as 

 anything better than a. possible mode of reducing conduc- 

 tion to a kind of electrostatics — an interchange of electric 

 charges among a series of conductors. If such a series of 

 vibrating and colliding particles existed, then certainly a 

 charge given to any point would rapidly distribute itself 

 over the whole, and the potential would quickly become 

 uniform ; but it by no means follows that the actual 

 process of conduction is anything like this. Certainly it 

 is not the simplest mode of picturing it for ordinary pur- 

 poses. The easiest and crudest idea is to liken a wire 

 conveying electricity to a pipe full of marbles or sand 

 conveying water ; and for many purposes, though not for 

 all, this crude idea suffices. 



Leaving the actual mode of conveyance as unknown,, 

 let us review how much is certainly known of the process 

 called conduction. 



This much is certainly known : — 



(i) That the wire gets heated by the passage of a 

 current. 



(2) That no trace of a tendency to reverse discharge 

 or spring back exists. 



(3) That the electricity meets with a certain amount of 

 resistance or friction-like obstruction. 



(4) That this force of obstruction is accurately propor- 

 tional to the speed with which the electricity travele 

 through the metal — that is, to the intensity of the curri 

 per unit area. 



About this last fact a word or two must be said. T! 

 amount of electricity conveyed per second across a unit 

 area is called the intensity of current ; and experiment 

 proves, what Ohm originally guessed as probable from 

 the analogy of heat conduction, that this intensity i^; 

 accurately proportional to the slope of potential which 

 causes the flow ; or, in other words (since action and 

 reaction are equal and opposite), that a current in a 

 conductor meets with an obstructive electromotive force 

 exactly proportional to itself. The particular ratio be- 

 tween the two depends upon the particular material of 

 which the conductor is composed, and is one of the con- 

 stants of the material, to be determined by direct 



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