Researches on Electro -magnetism. 195 



way where the head of the nail joined the stronger magnet. In 

 1848, M. Dub of Berlin instituted an inquiry into the lift- 

 ing power of electro-magnets with reference to the form of the 

 attracted mass. This inquirjf proves that the term "lifting 

 power " can convey no definite idea, inasmuch as a mere change 

 of form in the mass attracted will cause the energy with which 

 the magnet grasps it to vaiy between the limits of 1 and 10^ or 

 perhaps between even wider limits. 



The apparatus used by Dub was a steelyard, from one end of 

 which a bar of soft iron " submagnet*^" was suspended vertically, 

 a weight being moved along the opposite arm until the bar was 

 torn from the magnet underneath. As early as 1833, Dal Negro 

 had asserted that the attraction of the magnet increased viath 

 the weight of the submagnet ; the first query which Dub applied 

 himself to was, to ascertain ivhether, when the mass of the latter 

 remains constant, the attraction remains constant also. Ten cylin- 

 drical bars were chosen, all of the same weight, but varying from 

 1 to 16 inches in length, and from 2 inches to i an inch in dia- 

 meter ; a stream, the strength of which is expressed by the tan- 

 gent of 25°, was sent round the magnet, and the weight neces- 

 sary to separate each bar from the magnet was determined. The 

 following table exhibits the results obtained, the weights being 

 expressed in pounds and decimals of a pomid : — 



I. 



Submag. No. 1. No. 2. No. 3. No. 4, No. 5. No. 6. No. 7. No. 8. No. 9. No. 10. 

 1-8 17 21 2-7 3-35 4-5 6-2 74 97 10-3 



When a stream of 45° strength was sent round the magnet, the 

 following weights were obtained : — 



Submag. No. 1. No. 2". No. 3, No. 4. No. 5. No. 6. No. 7. No. 8. No. 9. No. 10, 

 6-24 5-4 5-2 657 813 10-71 15-5 1915 279 25-58 



We have in all these cases the same quantity of iron ; but the 

 mere peculiarity of form is sufficient to make the attraction of 

 No. 9 upwards of five times that of No. 3. But there is another 

 circumstance worthy of notice. With a force of 45° the attrac- 

 tion of the thick bar No. 1 is about three and a half times 

 what it was with a force of 20° ; the attraction of the thin bar 

 No. 10 has not, however, increased in the same ratio ; with a 

 force of 45° it is only two and a half times what it was with a 

 force of 20°. 



* This term, expressive of the mass of iron which is attracted by the 

 magnet, and wliich, in its turn, attracts the magnet, is introduced here for 

 the first time. The word ' keeper ' sugf^ests an idea which has no reference 

 to the present case. The word ' anchor,' used in Germany, is also inapph- 

 . cable. As tiie mass of soft iron, when induced, is itself a magnet, the term 

 introduced above seems to be a|)propriatc. It suggests, not only the at- 

 tractive pr(j|)crty of the induced mass, but also its relatiou to the e.\citing 

 magnet. — J. T. 



02 



