\ 



TBANSACTIONS OF SECTION A. 



655 



.Mixtli piiwcr ol' lli« rurront. TIio filiuiient syocdily broakn. Tin* point of donnrliive 

 iVmu tilt' law indicates ii point wIumi a cliim^ri* of .stiitf occurs in llin carbon lilanicnt. 

 Di.siiili'firation probably M'ts in. 'I'liis puint oiij.'lil tu bi- dolcrininod lor cacli kind 

 i.f Iniiip, and it should ni'ver Ix' allowi'd to ha rt'aclu'd,l'or it is iVoni tlii^ point that 

 (Ifcay ioniniencL's and rupturo Ibllows. 



)W lamp 

 current 

 cenco is 

 isistancti 

 s occurs 

 lian the 



11'. On llic. T'ljitdficinii t>f Di/nanin.EIriiiUi'. }rnflilnci<. 



lUj Prol'ci-SOX' SlIA'ANUS P. TlIO.Ml'SoN. 



15. On Earth Currents. Bi/E. 0. WAt.Ki;i;. 



lv\ten(b(l ol)si'r\atioiis of thcso currents in Iiulia slmw potential to be east 

 dinin;.' the Jurenoons ninl west in the al'lernoons, that is to .say, currents will How in 

 tlie telef;raph lines from oast to west in the forenoon, and west to east in the after- 

 noon. The phenonieiion .seems to bo a very invariable one on quiet days, both with 

 reirard to times of mnxinia and times of chan;.'i'. 'J"he maxinui appear to occur 

 ;il)i)ut !» to 10 A.M. and -' to ."i r.jr. 



The fact of the oxistence of these currents is often attributed to the earths 

 niiignetic field, and no doubt sensitive instruments would revenl such induce(l 

 carrentM, nnderlyin<; tho.se ])r()duced by more local causes. The author thinks that 

 the variable currents most frenerally noticed are duo to the latter. Sonu'thinL'' elso 

 than the mere ])assaf;i! of a wire ihrouizh a tolerably uniform ma^nietic Held i., 

 required to explain observed facts. That elevation of one place above another will 

 jfive a ])ermanent dillerence of potential between the two, the amount varying- 

 aopordinj,'- to circumstances, l)ut observed to beat least, O'J volt per 2, 000 feet. 

 That the current between two stations in land will chanye its direction an liuur or 

 two earlier than that between the westerly of those stations and a coast station 

 west. That a jreneral rainfall will diminish potential. That two ])laces situated 

 (in the same lineof telee;rnph,in the same latitude, n\ay have with reirard to another 

 station east or west, the one a hiirber, the other a lower ])otential than the latter. 

 f^ivin<( rise in tin; same wire cut at ar. intermediate point, in the (ino case to a current 

 east to west, and in the oth"i case to a current west to east. 



All these facts show that the currents usually observed are not created ]iy the 

 earth's maixnctic force. Doubtless, in temperate ref^ions, such lar^je dillerenci'S of 

 potential are not experienced as in tlie tropics, and other causes may contribute to 

 render the currents more variable. Hut it is in these, where the sun's rays aro so 

 nuich more powerful, and where irreprular meteoroloirical ])henomena do not occur, 

 that observers have the advanta;.'e day by day of witnessinfr dill'erence of terrestrial 

 potential in a more intense dt>;iTee, and with such refrularly recurrent channesa.s to 

 leave; no doubt on the mind as to their ori<rin. It is ditlicult to frame a theory 

 that can meet all observed phenonenia, especially in the case of the currents 

 observed in submarine cables which, it is bnyely af^reed, reverse their direction 

 witli the chanf):o of tide. It is thoup-ht, however, that if the relative differences of 

 temperature and humidity at the terminal stations bad been always noticed, the 

 same conclusion would have betni renched which is forced upon the observer in a 

 tropical country, viz,, that the dilii'rence of iiotential is simply due to the rapidity 

 with which evaporation is takintj- place at different jilaces. bivaporation leaves a 

 positive charfTO on the earth's surface ; should this char;.'c at A be of frreater tension 

 than at 13, and these two ])lace3 be put into connection i>y a wire, there will be a 

 tendency to equalise the two charges, and a resultant current from A to 15, which 

 will be sustained as lonpr as evaporation is proceedinj^- more rapidly at A than at B. 

 Such electric charges would produce no currents in the crust of the earth, and it is 

 yet to be proved that such exist. According- to this theory the greatest dill'erence 

 of potential would be obtained between two j)laces far apart on the earths' .surface, 

 where atone place the burning rays of the sun were falling, say on the shore of .«ome 

 intertropical sea, and where at the' other the still humid night was in the ascendant. 

 So far as a judgment is able to be formed by the comparatively limited area — 

 900 miles — observed by the author, it appears that this is borno out : — 900 miles 



