486 Mr. S. Bidwell. On the Lifting Power of [June 10, 



VI. " On the Lifting Power of Electromagnets and the Mag- 

 netisation of Iron." By Shelford Bidwell, M.A., LL.B. 

 Communicated by Lord Eayleigh, Sec. R.S. Received 

 June 1, 1886. 



I believe that no very recent investigations have been made with 

 reference to the maximnm lifting power which an electromagnet is 

 capable of exerting, and the experiments conducted by Joule between 

 the years 1839 and 1852 still form the basis of most of our practical 

 knowledge on the subject.* 



It is a matter of common experience that if an electromagnet be 

 excited by a gradually increasing current a limit is soon reached, 

 beyond which the ratio of increase of sustaining power to increase of 

 current becomes rapidly smaller ; and it has generally been assumed 

 that this ratio continues to diminish indefinitely, so that an infinite 

 current would not impart to a magnet much greater lifting power 

 than that which it possesses when an approach to saturation is first 

 indicated. 



Joule, after having shown by experiment that the power of an 

 electromagnet varies as its sectional area, expressed the opinion that 

 " no force of current could give an attraction equal to 200 lbs. per 

 square inch," f and much later Rowland stated, J as a probable result 

 of his well-known researches in magnetic permeability, that the 

 greatest weight which could be sustained by an electromagnet with an 

 infinite current was, for good but not pure iron, 177 lbs. per square 

 inch, or 12,420 grams per square centimetre of section. 



It has long been known that when an iron rod is magnetised its 

 length is in general slightly increased. Some experiments on this 

 effect of magnetisation, an account of which has been given in two 

 papers recently communicated to the Royal Society, § show that if the 

 magnetisation is carried beyond the point at which the magnetic 

 elongation of the rod reaches a maximum, the length of the rod, 

 instead of remaining unchanged, steadily diminishes, the curve ex- 

 pressing the relation between the length and the magnetising force 

 descending in a perfectly straight line which within the limit of the 

 experiments shows no tendency to become horizontal. Some further 

 experiments (not yet published) have also been made with rings of 

 iron instead of rods, and effects of precisely the same character were 



* Physical Society's Eeprint of Joule's Papers; also "Phil. Mag.," ser. 4, vol. 3 

 (1852), p. 32. 



t The greatest attraction which he succeeded in actually producing was 175 lbs. 

 per square inch. 



% " Phil. Mag.," ser. 4, vol. 46 (1873), p. 140, and vol. 48 (1874), p. 321. 

 § " Proc. Eoy. Soc," vol. 40 (1886), pp. 109 and 257. 



