288 CARNEGIE INSTITUTION OF WASHINGTON. 



to the motion, or is two-dimensional, like the magnetic field. Within the 

 magnets themselves the total electromotive intensity is zero and there is an 



electric polarization - [vl] at points where the intensity of magnetization is /. 



It is immaterial whether the intensity is calculated from [Bv], or whether it 

 is calculated from — V(Av) ; or we may consider that the effect of the polariza- 

 tion is exactly neutralized by that of the equal and opposite polarization due 

 to the charges induced on the parts of the conductor adjacent to the individual 

 magnetons, and that the net electric field remaining is due to the electric 



displacement produced by the motional intensity - [vB]. In connection with 



this case, approximately realized in one of my experiments, Swann has stated 

 that Maxwell's equation for the electromotive intensity can not be immedi- 

 ately applied to the case of rectilinear motion to show that the field is polar, 

 because in this case the vector potential is not independent of the time. This is 

 clearly an error. Several examples of the contrary are given above in addi- 

 tion to this particular case. 



VIII. A conducting solid of revolution magnetized along its axis (either per- 

 manently or inductively or both) and in steady rotation about this axis. — Joch- 

 mann in 1863 and Larmor in 1884, referring to Jochmann, gave the general 

 form of the solution and worked out all the details for the case in which the 

 solid is a sphere. Recently Swann, who does not refer to the earlier work, 

 has again given most of the details for the sphere. The field is a purely 

 polar field arising from the superposition of various parts which are discussed 

 in detail. Maxwell's theorem can not be applied directly to the complete 

 rotating system, but it can be applied to each element, which has its own 

 linear velocity and vector potential. 



Some remarks on electromagnetic induction. S. J. Barnett. 



This paper is devoted chiefly to historical and critical comments on matters 

 connected with experiments previously made by the author in the field of 

 electromagnetic induction. 



Experiments on the motion of insulators in magnetic fields by Faraday, 

 Blondlot, H. A. Wilson, H. A. and M. Wilson, and the author are referred to 

 and the general theory is given in detail. H. A. Wilson considered his experi- 

 ments to prove that the motional electric intensity or electromotive force is 

 proportional to (K—l), which is not correct. According to all theories the 



motional intensity is independent of the medium and equal to - [vB], while 



c 



on the theory of Larmor and Lorentz the resulting polarization is proportional 

 to (K—l), the result supported by all the experiments. 



Wilson's procedure is as follows : He begins with the (erroneous) assump- 

 tion that the Larmor-Lorentz theory requires that the electromotive force 

 [F 1 ] in an insulator "should be equal to the electromotive force [F] in a con- 

 ductor multiplied by the factor (1— K~ l ), where K is the specific inductive 

 capacity." But when he comes to formulate his equations, he makes the 

 electromotive force F, not F' ', as required by his assumption, act on the moving 

 part of the insulator. Then, going back to his fundamental assumption, he 

 calls the quantity (1 — K~ l ) F, proportional to the effect observed, the electro- 

 motive force, while this is really F, and hence draws the conclusion: "The 

 amount of the displacement agrees with that calculated on the assumption 

 that an electromotive force is induced in the dielectric equal to that in a con- 

 ductor multiplied by (1 — K~ 1 )." The correct conclusion is: On the assump- 

 tion that the motional intensity or electromotive force is the same in all 



