STUDIES ON MAGNETIC DISTRIBUTION 



117 



At present let us take the case of electromagnets. 



For observing the effect of the permeability, I took two wires 12-8 

 inches long and -19 inch in diameter, one being of ordinary iron and 

 the other of Stubs' steel of the same temper as when purchased. These 

 were wound uniformly from end to end with one layer of quite fine 

 wire, making 600 turns in that distance. 



In finding / from Q" f) the latter was divided by 4~JL, except at the 

 end, where the end-section was included with JL in the proper manner. 

 x was measured from the end of the bar in inches. 



The observations in Table XI are the mean of four observations 

 made on both ends of the bar and with the current in both directions. 



TABLE XI. 

 IKON ELECTROMAGNET. 



The agreement with the formula in this Table is quite good; but we 

 still observe the excess of observation over the formula at the end, as 

 we have done all along. Here, for the first time, we see the error 

 introduced by the method of experiment which I have before referred 

 to (p. 98) in the apparently small value of 4;rA at x= -75. 



On trying the steel bar, I came across a curious fact, which, how- 

 ever. I have since found has been noticed by others. It is, that when 

 an iron or steel bar has been magnetized for a long time in one direction 

 and is then demagnetized, it is easier to magnetize it again in the same 

 direction than in the opposite direction. The rod which I used in this 

 experiment had been used as a permanent magnet for about a month, 

 but was demagnetized before use. From this rod five cases of distribu- 

 tion were observed: first, when the bar was used as an electromagnet 

 with the magnetization in the same direction as the original mag- 



