1 66 Perkins — Velocity of the Propagation of Magnetism. 



10 cm and the core a bundle of 64 small steel wires, up to 96° 12' 

 in 10 cm for a steel bar of l'2 cm diameter. This shows at least 

 qualitatively what we should expect, that lamination reduces 

 the lag. The curves of flux intensity taken along the bar 

 to be logarithmic curves whose equation was Q = 

 /3 varied somewhat, and his results were tabulated as 



appeared 

 follows : 



/3 for soft iron = 



10-27 

 10*17 



10-07 



(3 for hard iron == \ 14- 



for steel 



14-51 

 16-16 

 i6-37 



x being measured in meters. In conclusion, the author says that 

 the size of yS varies only as the material and is independent of 

 the diameter. This is also true of direct current phenomena ; 

 hence he argues that were it not for the opposition of eddy 

 currents the velocity would be enormous. 



When I first began investigations on this effect I was not 

 aware of the work just cited. But inasmuch as the results 

 were somewhat of a qualitative nature, and the method used is 

 open to serious objection, I thought it worth while to continue 

 the research. 



The method employed was in some respects similar to that 

 of Oberbech. A magnetizing coil was mounted on the 

 center of a steel bar about one meter long and having a section 

 of 2'83 cm2 . It was of the best English tool steel. This material 

 was used in order to obtain as low a velocity as possible and 

 thus measure it more readily. On the middle of the magnetiz- 

 ing coil were wound 250 turns of fine wire, and sliding on the 

 core were two more coils, one of 500 turns and one of several 

 thousand. As will be seen, the exact number is not an essential 

 quantity. 



In performing the experiment one of the movable coils was 

 set at some known distance from the exciting coil at the center, 

 and the angle of lag between it and the small coil of 250 turns 

 was measured. For short distances, up to about 8 cm , one of 500 

 turns was used ; beyond that the larger coil. The instrument 

 by which this angle was measured was a small quadrant elec- 

 trometer having double quadrants and an aluminum needle 

 suspended by a quartz fiber and dipping into sulphuric acid 

 through which the contact was made. The needle and one pair 

 of quadrants constituted one terminal and the other pair of 



