Wave Propagation of Magnetism. 377 



by the opposition of poles of the same or contrary sign, is to 

 place one of the small coils on the core of a large coil, at one 

 end of this latter coil, and to place the other small coil on a 

 small rod of iron or steel, and to bring this rod with its coil 

 near one end of the core of the large coil and afterwards to 

 the other end. The change of phase of the ellipses is readily 

 shown, and is evidently dne to the threading of the lines of 

 force by induction through the small coils in the same or in 

 opposite directions. 



If an incandescent lamp of about one-candle power is 

 connected with one of the small coils, and the coil is placed on 

 the iron ring between poles of the same name, the lamp will 

 not glow ; but on changing the poles so that two of opposite 

 sign shall be opposed, the lamp immediately glows. This 

 point, however, cannot be called a true nodal point, and I 

 failed to find any other. Although it is probably true that 

 the most distant molecule of the iron quivered under the 

 periodic alternations of magnetism to which it was subjected, 

 yet there was no wave-motion along the iron ring. Just as 

 a distant molecule might have responded quickly to the first 

 application of heat to the ring, there is no true wave-motion 

 in the propagation of the heat. It seems to me that the pro- 

 pagation of magnetic disturbances produced by forced oscilla- 

 tions on iron bars is closely analogous to the propagation of 

 heat over these bars, and that each molecule of the iron swings 

 under the directive moment of the magnetic field somewhat 

 like a pendulum in molasses. 



Instead of the formula 



we have 



d 2 u 7 du „ ~ 



^72- + ^3T +nru = h COS pt, 



dt 2 dt 



The curve of distribution of magnetism on the ring can be 

 made to agree closely with an exponential form, which is 

 evidently the solution of the second of the above equations. 



The projection of the ellipses on a screen makes a very 

 interesting lecture-experiment. 



To ascertain whether changes of phase could be detected 

 between the inner portions of a thick iron core subjected to 

 alternating magnetic impulses and the outside of this core, I 

 placed one of the coils of the phasemeter in a channel which 

 was turned about the centre of a core of iron, the diameter of 

 the curve being 2J- inches. Another coil with its centre 



