Prof. Norton on Molecular Physics. 283 



Electro-magnetism. — The essential theory of the development 

 of magnetism by electric currents, and of electro-magnetic phe- 

 nomena generally, is embracd in what has already been stated 

 (pp. 278-281). 



Magneto-electricity. — The excitation of electric currents by 

 magnetic action is a phenomenon of pure induction, and will be 

 included under the next head. 



Induction of Electric Currents. — The term induction, as ordi- 

 narily used in dynamical electricity, has reference only to the 

 development of currents, through wires, or upon the surfaces, or 

 within the mass of bodies. It will suffice to confine our atten- 

 tion to the origination of currents in wires forming a closed 

 circuit. The general idea of the process, as contemplated from 

 our theoretical point of view, has already been indicated. The 

 fundamental principle which covers the whole ground is, that 

 whenever the action of the impulsive force of any current begins 

 or is on the increase, the induced current sets in the opposite 

 direction to the inducing current ; and when the same action 

 terminates or is on the decrease, the induced flows in the same 

 direction as the inducing current. The rationale is, that the sethe- 

 real pulses of the originating force fall upon the condensed sether 

 at the central portion of each group of molecules, and urge out- 

 ward the interstitial electric sether on that side of the group. 

 This determines a flow of a certain portion of the electric sether 

 around to that side, and a consequent positive polarization on 

 that side of the group or compound molecule. While this 

 process of polarization is going on, there must then be a flow of 

 electricity from one compound molecule to the next, in the same 

 direction in which the transfer of eether occurs within the mole- 

 cules themselves, as an inevitable attendant upon the increasing 

 polarization (p. 101). When the primary action becomes con- 

 stant, there simply abides a state of static polarization, an 

 " electrotonic state," in which an equilibrium prevails through- 

 out the circuit. But when this action falls off, the polarization 

 maintained by it must decline, and a current arises setting in 

 the opposite direction to the former one. The inducing cur- 

 rent may be either electric or magnetic, so called. The in- 

 crease of its action upon the adjacent wire may result either 

 from the closing of the circuit in the case of a galvanic current, 

 or the magnetization of the iron when a magnet is employed ; 

 or from the approach of the current to the wire; or from a 

 change in the position of the wire in the magnetic field, attended 

 with an augmentation in the action of the external impulsive 

 force exerted by all the circular currents of the magnet. A de- 

 crease in the polarizing effect of the inducing current may result 

 from an interruption of the circuit, or a diminution in the force 



