TRANSACTIONS OF SECTION A. 655 



.sixth power of the current. The filament speedily breaks. The point of departure 

 from the law indicates a point when a change of state occurs in the carbon filament. 

 Disintegration probably sets in. This point ought to be determined for each kind 

 of lamp, and it should never be allowed to be reached, for it is from this point that 

 decay commences and rupture follows. 



14. On the Equations of Dynamo-Electric Machines. 

 By Professor Silvancs P. Thompson. 



15. On Earth Currents. By E. 0. Walkek. 



Extended observations of these currents in India show potential to be east 

 during the forenoons and west in the afternoons, that is to say, currents will flow in 

 the telegraph lines from east to west in the forenoon, and west to east in the after- 

 noon. The phenomenon seems to be a very invariable one on quiet days, both with 

 regard to times of maxima and times of change. The maxima appear to occur 

 about 9 to 10 a.m. and 2 to 3 p.m. 



The fact of the existence of these currents is often attributed to the earth's 

 magnetic field, and no doubt sensitive instruments would reveal such induced 

 currents, underlying those produced by more local causes. The author thinks that 

 the variable currents most generally noticed are due to the latter. Something else 

 than the mere passage of a wire through a tolerably uniform magnetic field is 

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

 give a permanent difference of potential between the two, the amount varying 

 according to circumstances, but observed to be at least 02 volt per 2,000 feet. 

 That the current between two stations in laud will change its direction an hour or 

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

 west. That a general rainfall will diminish potential. That two places situated 

 on the same line of telegraph, in the same latitude, may have with regard to another 

 station east or west, the one a higher, the other a lower potential than the latter, 

 giving rise in the same wire cut at an intermediate point, in the one case to a current 

 east to west, and in the other case to a current west to east. 



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

 earth's magnetic force. Doubtless, in temperate regions, such large differences of 

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

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

 much more powerful, and where irregular meteorological phenomena do not occur, 

 that observers have the advantage day by day of witnessing difference of terrestrial 

 potential in a more intense degree, and with such regularly recurrent changes as to 

 leave no doubt on the mind as to their origin. It is difficult to frame a theory 

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

 observed in submarine cables which, it is largely agreed, reverse their direction 

 with the change of tide. It is thought, however, that if the relative differences of 

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

 same conclusion woidd have been reached which is forced upon the observer in a 

 tropical country, viz., that the difference of potential is simply due to the rapidity 

 with which evaporation is taking place at different places. Evaporation leaves a 

 positive charge on the earth's surface ; should this charge at A be of greater tension 

 than at B, and these two places be put into connection by a wire, there will be a 

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

 will be sustained as long as evaporation is proceeding 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 difference 

 of potential would be obtained between two places far apart on the earths' surface, 

 where at one place the burning rays of the sun were falling, say on the shore of some 

 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 borne out : — 900 miles 



