346 TENTH ANNUAL REPORT OP 



one half of which passes through the multiplier, and the power of the 

 current passing through this instrument is now only 



E 



but the needle can he restored to its original position by suitably dimin- 

 ishing the resistance K by means of the rheostat. If by turning it, 

 the resistance of the undivided part of the circuit is reduced from 

 R to I R, the strength of the current is 



S" = 



1 E _ E 



2 



Ili+lgr K+gr 



2 



therefore it is again as strong as at first. Hence, if after the insertion 

 of the brancli wire v, a number n of coils of the rheostat must be taken 

 out of the circuit to recover the original deflection of the needle, then 

 the resistance R of the undivided part of the circuit is equal to that 

 of 2 ?i coils. 



But the resistance R consists of two parts — the essential resistance 

 of the element, and the resistance of the conducting Avire from one pole 

 to a, and from the other to b. The resistance of these wires has to be 

 determined and subtracted from R to find the essential resistance of 

 the element. 



This, as well as all other indirect methods for determining the es- 

 sential resistance of an element, is not so simple that it should be 

 preferred to the direct determination described above, if an instrument 

 for measuring the force of current is at command. 



§ 13. Poggendorff's method for determining tlie electro-motive force of 

 inconstant batteries. — In volume LIII of his Annals, page 436, Pog- 

 gendorflf communicates his first experiment on the electro-motive force 

 of the zinc and iron battery. ' Although iron is much nearer to zinc in 

 the tension series than copper, yet the current which the combination of 

 zinc and iron produces in dilute sulphuric acid, is stronger than the 

 current of an element of copper and zinc in the same liquid and under 

 like circumstances. 



This result at first glance appears to be in opposition to the contact 

 theory; hence Poggendorff" undertook a more exact investigation. He 

 determined, as well as it is possible with the changeable current of bat- 

 teries with one liquid, the resistance and the electro-motive force of both 

 combinations, by Ohm's method, and found, that in fact the electro- 

 motive force of the zinc and iron battery was to that of the zinc and 

 copper as 21.5 to 11.8. 



Thus the electro-motive force of the zinc and iron battery is actually 

 greater than tliat of the zinc and copper, though in the tension series, 

 iron stands between zinc and copper. Poggendorff" saw that the cause 

 of this anomaly could only be the polarization of the plates. The 

 electro-motive force, which originally set the current in motion, is 

 limited by the electrical diff'erence of the metals in contact; but as soon 

 as the current begins to circulate, the metal-plates undergo a polar- 

 ization w^hicli diminishes the original electro-motive force, and this 



