254 M. E. Edlund on the Heat disengaged 



sion between the two currents. If, on the contrary, the conduc- 

 tor is removed, the induced current is in the opposite direction, 

 from which would result an attraction between the two currents. 

 Induction, therefore, in the two cases is accompanied by an ex- 

 penditure of mechanical force. If, on the contrary, the galvanic 

 induction is produced by a change in the intensity of the indu- 

 cing current, without any change in the relative positions of the 

 two currents, there is no loss of mechanical force. In this 

 respect, then, the two kinds of current are dissimilar. At first 

 sight it would appear that an approximation of the two currents 

 ought to have the same effect as an increase in the intensity of 

 the inducing current, and that their separation would have the 

 same result as a diminution in the intensity of the inducing cur- 

 rent — in fact, that approaching the induced circuit to, or remo- 

 ving it from, the inducing current, would really only produce an 

 increase or a decrease in the intensity of the latter. But if the 

 principles of the mechanical theory of heat can be applied here, 

 when the induction is accompanied by an expenditure of mecha- 

 nical force, the development of heat must necessarily be greater 

 than when the induction arises from a change in the intensity of 

 the current ; and this excess must be proportional to the work 

 performed. 



Induction-currents offer, therefore, a suitable means for con- 

 firming experimentally the general validity of the mechanical 

 theory of heat. 



These considerations led M. Edlund to study more closely 

 than had hitherto been done the thermal effects of induction- 

 currents. We shall give a brief analysis of his paper, in which 

 he commences by stating the laws of induced currents, and passes 

 then to the relation which exists between the heat produced and 

 the mechanical force exerted. 



II. 



An induction-current can scarcely have any other properties 

 than those of an ordinary galvanic current whose intensity is 

 constantly changing. It was easy to foresee, therefore, that the 

 heat which it produces at a given time must be proportional to 

 the square of its intensity at the same time. The means hitherto 

 employed in regard to this question, when constant galvanic 

 currents were concerned, are insufficient for demonstrating the 

 perfect accuracy of this proposition in the present case. On the 

 other hand, Weber's electrodynamometer furnishes a very simple 

 method for this object. ' This instrument, as we know, consists 

 of two coils of silk-covered copper wire, one of which is fixed, and 

 the other suspended to two fine silver wires so as to be capable of 

 oscillating freely about the position of equilibrium determined by 



