338 PROCEEDINGS OF THE AMERICAN ACADEMY 



qualitatively are the two sets of phenomena alike, but also, to a large 

 extent, quantitatively ; and there are reasons more than sufficient to 

 account for their differences. We are led, therefore, to conclude that 

 the laws of magnetic propagation are in all probability the same as, 

 or very similar to, those of the establishment of the electric current ; 

 and consequently to the corresponding laws of heat. 



SUMMARY. 



I. The magnetic waves induced in a bar of soft iron by an alter- 

 nating current differ in form, phase, and magnitude from the valuea 

 assigned to them by any theory of instantaneous propagation. 



II. They differ in form, inasmuch as the rise and fall of the mag- 

 netic wave is in no case abrupt ; but it presents, even for a moderate 

 number of reversals to the second, a perfectly rounded crest and 

 trough, especially at considerable distances from the primary. 



III. The magnitude of the waves is constant up to five or six rever- 

 sals per second, after which it is much lessened ; from twenty to thirty 

 reversals, it diminishes nearly with the number of reversals (the 

 deflection of the galvanometer remaining constant) ; from fifty to one 

 hundred and forty it falls off more and more rapidly, especially for 

 moderate distances along the bar, being there reduced so as to give 

 about one thirtieth of its former maximum deflection. 



IV. The phase within the primary is retarded more and more as 

 the rapidity of alternation of the current increases, — to a maximum 

 of 90°, and for a distance not too remote is invariably later than it is 

 within the primary. 



V. The apparent retardation of the phase depends upon the period 

 (length) of the wave ; the arbitrary distance from the primary ; the 

 material, dimensions, and special condition of the bar ; and even, in 

 extreme cases, upon the strength and distribution of the magnetic 

 field. 



VI. The retardation increases for a certain distance (about thirty 

 diameters) to a maximum of from 127°, to 130° ; or from 37° to 40° 

 later than that of the primary ; falling, again, gradually, at great 

 distances, to 90°, that is to the original value within the primary 

 itself 



VII. The apparent velocity of propagation, accordingly, increases 



