Intelligence and Miscellaneous Articles, 155 



it proves also that the two effects of the machine and the battery 

 are superposed and independent. 



If we now suppress the battery, but iguite in the circuit a burner 

 formed of two unequal carbons — one stout (0-004 metre), the other 

 thinner (0-002 metre), — this asymmetry suffices to determine a 

 permanent deflection of the compass, just as if a battery had been 

 intercalated. The two systems of mutually inverse currents given 

 by the machine cease, therefore, to be equal ; that which is directed 

 from the thick carbon to the thin, from the less hot part to that 

 which is more so, prevails over the system of which the direction is 

 opposite. A differential current results, indicated by the compass, 

 and the more intense as the difference of thickness of the two car- 

 bons is more marked ; it reaches its maximum between a large mass 

 of retort-carbon, which becomes little heated, and a crayon termi- 

 nating in a fine point, which attains the highest temperature. The 

 same phenomenon is obtained, and in the same sense, when an arc 

 is produced between a mass of any metal and a carbon point. As 

 it is difficult to maintain the constancy of the arc, the intensity of 

 the differential current is very variable ; the following results must 

 be regarded as only a first approximation : — 



Lead. Iron. Carbon. Copper. Mercury. 



29° 30° 31° 60° 70° 



In general the differential current is weak or none when the arc 

 is but of little extent ; it increases with the distance of the elec- 

 trodes. For zinc it is at first as intense as with copper ; but it 

 falls suddenly, probably on account of the oxide with which the 

 metal is soon covered. 



The deflection depends on two things : — 1, on the mean electro- 

 motive force of the differential current ; 2, on the resistance intro- 

 duced into the current by the arc which is formed. It is easy to 

 compare that force and that resistance in the different cases in the 

 following manner : — 



We introduce into the total circuit a battery of n pairs, having 



an electromotive force »A. According to whether it acts in the 



direction of the differential current or in the opposite direction, we 



have 



. _ x -j- «A ., _ x —nk. 

 % __, % _ 



"When x is greater than «A both deflections have the same sign. 

 and we find 



i -i cc+nA . Z.- + 1 



bnt if x is less than wA, the two deflections have opposite signs — 



x=nA- -. 



fc + 1 



