demonstrating a limit to the Magnetizahility of Iron. 453 



— that the maximum power of the electro-magnet^ j^directlpmo*^ 

 portional to its least transverse sectional area*^ ": ';' . >. ^f.r'?f 



Table IV. 



Least sec- 

 tional area in 

 square inches. 



Bfaximum 

 lifting power. 



Maximum 



power divided 



by area. 



[•No.l 



My own electro-magnets. < ^^ g 



1No!4! '.''''. 



Electro-magnet made by Mr. Nesbit. Length 

 round the curve 3 feet ; diameter of iron 

 2| inches ; sectional area 5*7 inches ; do. 

 of armature 4*5 inches ; weight of iron 

 about 50 lbs 



Professor Henry's : of iron 2 inches square, 

 the sharp edges being rounded ; length 

 round the curve 20 inches ; weight 21 lbs. 



Mr. Sturgeon's : length about I foot ; dia- 

 meter half an inch 



10 



0-196 

 0-0436 

 00012 



4-5 



3-94 

 0196 



2090 

 49 

 12 

 0-202 



1423 



750 

 50 



209 

 250 

 275 

 162 



317 



190 

 255 



The results of the table are, I think, sufficient to prove the 

 rule, if we allow for various sources of error. No. 1 is unfortu- 

 nately made of a piece of unsound iron, and was in all probabi- 

 lity not fully saturated, otherwise I have no doubt that its power, 

 per square inch section of the ferruginous circuit, would have ap- 

 proached 300 lbs. Again, the specific power of No. 4 is less than 

 the mean, simply on account of the extreme difficulty of making 

 a good experiment with it. With regard to Mr. Nesbit^s electro- 

 magnet, the battery used was so powerful (nineteen of DanielFs 

 two-feet cells) and the quantity of conducting wire so very large 

 (fourteen lengths of wire, each 70 feet long and about xV^^ ^^ 

 an inch thick), that its magnetism must have been brought to 

 the utmost possible pitch of intensity, which therefore exceeded 

 the mean specific power of the table. On the other hand. Pro- 

 fessor Henry^s, which was excited only by a single pair, could 

 not have been nearly saturated. 



The mean of the specific powers of No. 2, No. 3, and Mr. 

 Nesbit^s electro-magnet, may, I think, be fairly taken for the 

 expression of the maximum magnetic force of iron under ordi- 

 nary circumstances, which may therefore be taken as equal to 

 the least sectional area of the magnetic circuit in square inches, 

 multiplied by 280 lbs*. 



With regard to the magnetizing coils, I may observe that each 



* According to this, the maximum magnetic attraction of two iron sur- 

 faces, each 1 inch square, for one another will be 140 lbs. — May 1861, 

 J. P. J. 



