Dkcembee 27, 1895.] 



SCIENCE. 



875 



tMs integral current. Whatever is true of 

 the condenser as a whole is true of any 

 elementary part of the dielectric. Hence, 

 whenever a current passes through any 

 part of a dielectric it produces there a 

 change of state which we call polarization 

 and a consequent electromotive reaction 

 which is proportional to the total current 

 that has passed through. This total cur- 

 rent Maxwell calls total electric displace- 

 ment, selecting this name evidently for the 

 purpose of bringing out the strong resem- 

 blance of the relation just described to the 

 relation between the elastic compression of 

 a material body and the elastic reaction 

 produced thereby. The electric displace- 

 ment depends also on the nature of the di- 

 electric. Thus, the integral current be- 

 tween the same plates and acted upon by the 

 same cell will be greater if the plates are 

 separated by glass than if the intervening 

 space is a perfect vacuum. The ratio be- 

 tween the two is the specific inductive ca- 

 pacity of the glass. This constant is within 

 wide limits independent of the charging 

 electromotive force and it corresponds to 

 the elastic constant in elasticity. We have, 

 therefore, summing up these relations, the 

 following law of electric displacement or 

 flux: 



"Intensity of electromotive reaction in 

 any direction equals the intensity of electric 

 flux in that direction divided by the specific 

 inductive capacity." 



Second : A magnetized bar of iron is 

 magnetically polarized just as the dielectric 

 separating the plates of a charged condenser 

 is electrically polarized. The resemblance 

 between the two states is complete. We 

 can speak, therefore, of a magnetic flux 

 or displacement, just as we speak of an 

 electric displacement, and experiment tells 

 us that the first follows the same formal 

 law as the second, viz : 



Intensity of magneto-motive reaction in 

 any direction equals the intensity of mag- 



netic flux in that direction divided by the 

 magnetic specific inductive capacity or 

 permeability. 



It should be observed that no assumption 

 is made that these two physical constants 

 of the medium are the same in every direc- 

 tion. In an allotropic substance they can, 

 and generally will, be different in different 

 directions. 



The last law is not rigidlj' true for con- 

 ductors of high permeability like iron, 

 nickel, cobalt, bismuth, when the mag- 

 netizing force is high. The same limita- 

 ation exists in the deformation of elastic 

 bodies when the deformation passes beyond 

 the elastic limit. It is not a serious lim- 

 itation as long as we keep, as we neces- 

 sarily do in experimental investigations of 

 electric oscillations, within the limits of 

 what may be called the elastic limit of elec- 

 trification and magnetization. 



These two laws describe one of the two 

 essential elements in our modei-n view of 

 the electric and the magnetic force, that is 

 the view of these forces considered as reac- 

 tions of the dielectric against the continu- 

 ance of an abnoi'mal condition produced in 

 consequence of a certain process, called in 

 one case the electric and in the other the 

 magnetic current, having taken place there. 

 These reactions suffice to explain the at- 

 tractions and repulsions between electrified 

 and magnetized substances, which now ap- 

 pear not as direct actions at a distance, but 

 as a consequence of a definite distribution 

 of reactions in the dielectric separating the 

 bodies under considei-ation. These laws of 

 electric and magnetic flux occupy in the 

 modern electro-magnetic theory the same 

 position and have the same physical signifi- 

 cance as the laws of elasticity in mechanics 

 of a material body. This very important 

 element, we may call it the statical element, 

 our modern view of the electric and mag- 

 netic force was first clearly brought out by 

 Maxwell. His failure to illustrate it in a 



