378 Wave Propagation of Magnetism. 



coinciding with the centre of the imbedded coil was slipped 

 on the outside of the core : no changes of phase conld be 

 observed between the inner portions of the iron core and the 

 outside portions. When an armature was placed over the 

 imbedded coil, thus completely closing it in iron, hardly any 

 lines of force were threaded through it. They were diverted 

 to the surface of the iron core. Here we had the case of 

 Thomson's marine galvanometer, and the amplitude shown by 

 the telephone-diaphragm connected with the imbedded coil 

 was reduced to almost nothing. When, however, the arma- 

 ture which extended over the entire cross section of the core 

 was removed and an armature was employed which extended 

 the iron which passed through the centre of the imbedded 

 coil beyond the limits of the ends of the thick core, the lines 

 of magnetic force were brought down from the surface of the 

 iron and made to thread themselves through the imbedded 

 coil. Properly chosen armatures can be made to closely 

 resemble arrangements of lenses, bringing magnetic lines into 

 bundles or spreading them. An incandescent lamp can thus 

 be made to glow when it is connected with a coil imbedded in 

 iron, by diverting the lines of magnetic induction through it 

 with suitable armatures. To illustrate the distribution of the 

 lines of force about an electromagnet, one can employ the 

 phasemeter to advantage. By reducing the diameter of the 

 iron ring on which the coils I have described are slipped, so 

 that a sufficient number of lines of force are threaded from the 

 north pole to the south pole of one of the large coils, one can 

 show that an incandescent lamp can be lighted even when it 

 is outside the coil in a plane perpendicular to the axis of one 

 of the large coils and passing through its centre. It is 

 evident that there would be no induction through the coil if 

 the iron did not make a magnetic circuit. 



The experiments I have described were conducted with an 

 alternating machine giving currents of the period of 2500 to 

 5000 a minute. Doubling the rate of alternation only pro- 

 duced changes in amplitude. 



Dr. Harold Whiting, in the paper I have cited, could find 

 no true nodes in the propagation of magnetism along bars, 

 and my instrument also shows none ; and my conclusion is 

 that which I have already given, namely that the propagation 

 of magnetic induction in iron and steel is expressed by the 

 equation of motion of molecular magnets in a resisting medium 

 rather than by the equation of a wave-motion. 



Jefferson Physical Laboratory, 

 Cambridge, Mass., U.S. 



