Perchloride of Iron enters as an element. 141 



inferior to Bunsen's, but superior to DanielPs. Without re- 

 placing the former, it may be preferred in many cases to the 

 latter. A Daniell's battery of equal surface did not show an 

 equal constancy above a deviation of 12°. 



In the latter, a gradual intermixture of the liquids by endos- 

 mose occurred more rapidly, and was attended with greater dis- 

 advantages than with the perchloride of iron circuit. For in 

 the sulphate of copper circuit, the ziuc covers itself by degrees 

 with copper, and loses thereby a part of its exciting force ; while 

 the perchloride of iron which gradually penetrates into the porous 

 cell, although it assists to dissolve some of the zinc, produces no 

 precipitate of iron on the surface of the zinc cylinder, and con- 

 sequently leaves its activity unchanged. 



In order to measure the force of the chloride of iron circuit, 

 I employed a method which is of easy execution, without being 

 on that account less accurate. It is founded on the proposition 

 which directly follows from Ohm's law, that " when, in electric 

 circuits which are subject to one and the same very great resist- 

 ance, alterations in this resistance occur, of such smallness that 

 they exert no perceptible influence on the strength of the cur- 

 rent, then the strengths of these various currents is directly pro- 

 portional to the electromotive force which produces them." 



In the Annalen der Chemie und Pharmacie, vol. lxxxvi. p. 1, I 

 described a tangent galvanometer with a long multiplication coil, 

 the fine copper wire of which offers a resistance to conduction, 

 which is measured by a German silver wire, 28,000 metres long 

 by 1*5 millim. thick. With this resistance, of itself very great, 

 others were combined, by rolling on long and thin wires, which 

 amounted to 2*9 to 3'1 times as much. Against this amount 

 of resistance, that of a constant element of ordinary size mani- 

 festly vanishes, for this is seldom greater than one or two metres 

 German silver wire. If, then, various constant elements, all of 

 inconsiderable resistance, are closed with that long wire, and the 

 multiplication coil united with it, we may assume that their elec- 

 tromotive forces are as the strengths of the currents which set 

 them in motion, and by which the needle of the tangent galvano- 

 meter is deviated. 



It is stated in the paper above referred to, under what con- 

 ditions and with i a what limits the tangents may be taken as a 

 satisfactory expression of the strength of the current. The 

 numbers are, however, only then sufficiently accurate to be com- 

 pared, when they relate to observations made simultaneously, or, 

 at least, almost so. For the intensity of the force by which the 

 deviated galvanometer needle is recalled to its position of rest is 

 subject to certain fluctuations, whose disturbing influence often 

 exceeds the limits of the accuracy to be obtained by this kind of 



