Febrvaky 1, 1895.] 



SCIENCE. 



135 



i-ate of variation of the magnetic flux 

 through the eircuit tlie hist relation leads to 

 the following final i-esult : The total electro- 

 motive force iuduceil in an infinitely small 

 circuit wliich moves through a variable 

 electromagnetic field is composed of three 

 parts. First, the electromotive force due 

 to rate or variation of the magnetic flux 

 tlirough the circuit and produced by the 

 time variation of tlic field itself. Second, 

 the electromotive force due to the rate of 

 variation of the magnetic flux through the 

 circuit produced by the motion of the circuit. 

 The third component of the induced electro- 

 motive force can be described as follows : 

 Suppose that permanent magnetic charges 

 are distributed in any way whatsoever 

 throughout the field. There is then a 

 transference of magnetic matter through 

 the moving circuit. We may call it the 

 magnetic convection current, following a 

 suggestion of Hertz ( Unters. neb. d. Aus- 

 br. der el. Kraft, p. 2(5.5). This magnetic 

 convection cuiTent is equal to the quantity 

 of magnetic matter contained in the volume 

 traced out per unit of time by the moving 

 circuit, and is proportional to the third 

 component of the induced electromotive 

 force. This component does not appear in 

 Maxwell's theory, so that the Hertzian 

 equations seem to be more complete than 

 those of Maxwell. 



Poincare recognizes in this quite a differ- 

 ence between Maxwell's presentation of the 

 electromagnetic theory and that of Hertz ; 

 bat this difference will evidently exist onlj' 

 if it is proved that a di.stributiou of perm- 

 anent magnetism, whose induction flux over 

 a closed surface is a constant, different from 

 zero, can exist. The physical meaning of 

 such a distribution is far from being clear, 

 and Poincare might have well devoted more 

 attention to the elucidation of this perplex- 

 ing feature of the Hertzian equations. On 

 this point the student \\ill do well to consult 

 Boltzniann (Vorles. iiber Maxwell's Theorie 



d. Elec. & d. Lichtes, II. Theil, IX. Vorles.). 



The second gi-oup of equations refers to 

 the magnetomotive force induced in a circuit 

 which is changing its position with respect 

 to a field of given distribution of electrical 

 force and it is shown that the total magneto- 

 motive force induced in an infinitely small 

 circuit in motion is composed of four compon- 

 ents. The fii-st component is proportional 

 to the rate of change of the flux of electric in- 

 duction whicli constitutes the conduction 

 current. The second component is propor- 

 tional to the rate of change of the flux of elec- 

 tric induction wliich constitutes the displace- 

 ment current. The third component is pro- 

 portional to the rate of change of the electric 

 flux due to the motion of the circuit, and 

 the fourth component is proportional to the 

 convection current of permanent electro- 

 static charges, corresponding to what w"as 

 called above the convection current of perm- 

 anent magnetism. There is, however, )io 

 difficulty of conceiving a permanent electri- 

 fication of the dielectiic such that the total 

 flux of its induction through a closed surface 

 should be difl'erent from zero, and, therefore, 

 the magnetomotive force induced by an elec- 

 trical convection current is a priori evidi-nt 

 as soon as the correctness of the fundamental 

 assumptions in the Faraday-Maxwell theory 

 is admitted. There is no diff"erence between 

 this second group of equations and those 

 given by JIaxwell. 



It is pointed out that the existence of the 

 third component was verified by Rowland's 

 experiments (Pogg. Ann. 15<S, p. 487), 

 and the I'xistence of the fourth component 

 by the experiments of Roentgen (AVied. 

 Ann. 3<5, p. 204) . The magnetomotive force 

 due to displacement currents was, of course, 

 first pointed out by the experiments of 

 Hertz. 



Next follows a beautifiil mathematical 

 discussion of the mechanical forces acting 

 upon a body which is moving through an 

 electromagnetic field. The foHowing tyjKiS 



