248 CONNECTION OF ELECTRICITY AND MAGNETISM. 



These electro-motor forces, induced by electro-dynamic action, sbow 

 the same character as the pondero-motor forces before mentioned. They 

 depend on motion, and the lines along which they act are closed lines 

 without end; for a magnet, losing its magnetism, induces electro-motor 

 forces in circular directions all around its axis. 



In spite of this iundamental difference, it has been proved by the experi- 

 ments of Mr. J. Prescott Joule and by the general theoretical deductions 

 of Sir W. Thomson and the author of this article, that all the known 

 effects of electro-dynamic action are subject to the great principle of con- 

 servation of energy, although a theoretical deduction of this universal 

 principle of nature can be given only for forces of the first class, which are 

 independent of motion, and which tend always to a final positionof equilib- 

 rium. It can be proved that when an electric currrent, by moving a mag- 

 net, does mechanical work, the current induced by the motion of the mag- 

 net alters the relation between the chemical processes going on in the bat- 

 tery and the heat evolved in the galvanic circuit, so that a part of the 

 chemical forces is not spent in the production of heat, but used for the 

 mechanical work of the electro-dynamic forces. 



There are two principal questions which have been discussed: the one 

 a question of fact ; the other a question of theory. 



The first is this : hitherto we knew with some degree of accuracy 

 the electro-dynamic actions of closed galvanic currents onlj^, viz, of cur- 

 rents which circulate along a closed line or a system of closed lines, and 

 have no end, where electricity would be obliged to stop and to accumu- 

 late. The investigations of A mpere. Gauss, Neumann, (senior,) Kirchoff, 

 W. Thomson, &c., have led to a highly-developed mathematical theory 

 of closed currents, which enables us to calculate their electro-dynamic 

 effi^ts for circuits of any length and form, and which has been compared 

 with actual experimental measurements of the highest degree of pre- 

 cision. 



But there exist also currents with ends, as for instance those produced 

 by the discharge of a Leyden battery in the wire which connects the 

 tin-foil coatings of the glass jars. These coatings are the ends of the 

 conducting wire, and they are separated by the insulating glass of the 

 jars. We know that the wire produces electro-dynamic effects during 

 the discharge, like a wire closing together the ends of a galvanic bat- 

 tery, and that the currents in such a wire go to and fro, oscillating 

 between the two tin-foil coatings; but we do not yet know experiment- 

 ally how far these electro-dynamic actions are modified at the place 

 where the conducting circuit is interrupted by the insulating glass. 

 This question may be of little practical importance, because the actions 

 in question cannot but be very feeble, and it will require great experi- 

 mental skill to make them visible. On the other hand, in order to define 

 the ultimate causes of electro-dynamic actions, it is necessary to know 

 with certainty the part which every linear element of a current contrib- 

 utes to its general effect, and the linear elements even of a closed 

 circuit are not closed lines, but lines with two ends. 



