158 Mr. H. A. Rowland on Magnetic Permeability. 



square inch, and for nickel 63,000 times, or 22-9 lbs. on the 

 square inch. For pure iron, however, I think it may reach 

 180,000, or go even above that. It is seen that one of the 

 Norway rings gave a very high result ; this is explained by the 

 following considerations. All the iron rings were welded except 

 this one, which was forged solid from a bar 2 inches wide and 

 then turned. Even the purest bar iron is somewhat fibrous ; and 

 between the fibres we often find streaks of scale lying length- 

 wise in the bar and so diminishing the section somewhat if 

 the ring be welded from the bar; when, however, it is forged 

 solid, these streaks are thoroughly disintegrated ; and hence we 

 find a higher maximum of magnetism for a ring of this kind, 

 and one approaching to that of pure iron. But a ring made in 

 this way has to be exposed to so much heating and pounding 

 that the iron is rendered unhomogeneous, and a tail appears to 

 the curve like that in Table III. It is evident that this tail 

 must always show itself whenever the section of the ring is not 

 homogeneous throughout. 



Hence we may conclude that the greatest weight which can be 

 sustained by an electromagnet with an infinite current is, for 

 good but not pure iron, 354 lbs. per square inch of section, and 

 for nickel 46 lbs. 



Joule* has made many experiments on the maximum sustain- 

 ing-power of magnets, and has collected the following Table, 

 which I give complete, except that I have replaced the result 

 with his large magnet by one obtained later. 



Table IX. 



Magnet belong- 

 ing to 



Least area 

 of section, 

 square inch. 



Weight 

 sustained. 



Weight sus- 

 tained — 

 least area 



Q. 



Mr. Joule, -j o' 



u 



Mr. Nesbit 



Prof. Henry ... 

 Mr. Sturgeon... 



10 

 •196 

 •0436 

 •0012 



4-5 



3-94 

 •196 



2775 

 49 

 12 



•202 

 1428 

 750 

 50 



277 

 250 

 275 

 162 

 317 

 190 

 255 



154700 

 147000 

 154100 

 118300 

 165500 

 128200 

 148500 



It is seen that these are all below my estimate, as they should 

 be. Eor comparison, I have added a column giving the values of 

 Q which would give the sustaining-power observed ; some of 

 these are as high as any I have actually obtained, thus giving 

 an experimental proof that my estimate of 354 lbs. cannot be 

 far from correct, and illustrating the beauty of the absolute 



Phil. Mag. 1851. 



