the Galvanic Current through Iron. 227 



tion of w had been accomplished. I nevertheless intend to 

 continue these trials. 



The preceding considerations disclose to view an interesting 

 analogy with some of those in dynamics and thermodynamics, 

 which I will briefly express. 



(1) The coefficient of elasticity is the ratio of an increment 

 of pressure to the lessening of volume produced. Simultane- 

 ously with the latter, however, there results a rise of tempera- 

 ture. According as this is balanced or not by any force, e. g. 

 by radiation or conduction, we obtain for the coefficient of 

 elasticity a less or a greater value. 



(2) Specific heat is the ratio of a quantity of heat to the rise 

 of temperature produced. But simultaneously with the latter 

 an expansion ensues. According as this is compensated by 

 an external pressure or not, we get for the specific heat a less 

 or a greater value. 



(3) The resistance of an electrical conductor is the ratio of an 

 electromotive force to the current generated. But, if the con- 

 ductor is magnetically polarizahle. at the same time a circular 

 magnetizing results. According as ice balance this by any ex- 

 ternal force (for instance, by a longitudinal magnetizing) or not, 

 we obtain for the resistance a less or a greater value. 



As the second proposition is reciprocal to the first, so a 

 fourth can be placed over against the third : it does not, how- 

 ever, belong here. 



Even Joule's law respecting the heating of the closed cir- 

 cuit cannot strictly hold good for iron ; or, more exactly ex- 

 pressed, if in Joule's formula 



W= const, i'ho 



we put, for the heat W developed and for the resistance w, 

 values directly given by observation, we must obtain for the 

 constant a value different according to the value of ?', but 

 always too high in comparison with other metals. Lenz has 

 observed the times necessary for equal developments of heat 

 with different current-intensities, resistances, and metals ; he 

 in fact found nearly constant numbers for the product ti 2 u\ 

 These numbers, however, cannot be quite constant. For the 

 theoretic deduction of the law presupposes the absolute con- 

 stancy of the quantity w during the process under considera- 

 tion ; but in reality this never takes place, because the resist- 

 ance is dependent on the temperature. It is only the electro- 

 motive force E that remains constant. Xow ; since Lenz's 



