1887.] On the Magnetisation of Iron in Strong Fields. 201 



of 240 gave 19,840 for the induction in a bar of mild Whitworth 

 steel, and 18,250 in a bar of wrought iron.* The corresponding 

 values of |j are 1563 and 1437 respectively. Probably the highest 

 magnetisation reached in any experiments of this class already pub- 

 lished is that found by Mr. Shelford Bidwellf in his experiments on 

 the tractive force between the halves of a divided ring electro-magnet. 

 For a force || of 585 he gives 19,820 as the value of $} (calculated 

 from the tractive force) in a wrought-iron ring. The corresponding 

 value of | is 1530. 



With cast iron, Dr. Hopkinson found (in a sample of grey iron) 

 10,783 for the induction produced by a force of 240. The corre- 

 sponding value of |j is 841. 



In the space between the pole-pieces of a strong electro-magnet we 

 have a field of force of much greater intensity than it is practicable 

 to produce by the direct action of the electric current. This field is 

 not well adapted for experiments whose object is to determine with 

 precision the relation of magnetisation to magnetising force, on 

 account of the distortion which it undergoes when the piece of iron 

 to be magnetised is introduced into it. It is, however, well suited 

 for experiments whose object is to determine how much magnetism 

 the metal can be forced to take up. 



For this purpose it is of course necessary that the cross-section of 

 the test-piece should be much smaller than the area of the pole-piece 

 faces. In the following experiments the electro-magnet consisted of a 

 pair of vertical limbs 25 cm. long, with cores 5 cm. in diameter, joined 

 at the bottom by a horizontal yoke, and furnished on the top with 

 pole-pieces, made of soft hammered scrap iron, in the form of rect- 

 angular blocks with plane faces, whose distance from each other 

 could be adjusted at will. The faces were 5*25 cm. square. The 

 magnet was wound with wire large enough to permit a current of about 

 27 amperes to be used for a short time. In the earliest experiments 

 the test-piece to be magnetised was a round cylinder of soft iron, with 

 flat ends 0'34 cm. in diameter and 1*3 cm. long. This was covered 

 with an induction coil, consisting of a single layer of fine wire, which 

 extended over the whole length of the piece. It was placed length- 

 wise in the centre of the field, with the pole-pieces just touching its 

 ends, and the field magnet was excited. The test-piece was then 

 suddenly withdrawn, while the transient current produced in the 

 induction coil was measured by a ballistic galvanometer connected to 

 the induction coil by long leading wires, which were twisted together 



* J. and E. Hopkinson have observed an induction of 20,000 in the core of a 

 dynamo -armature, under a force estimated at 740 (' Phil. Trans.,' 1886 (Part I) 

 p. 355). 



t S. Bidwell, " On the Lifting Power of Electro-magnets and the Magnetisation 

 of Iron," ' Roy. Soc. Proc,,' vol. 40, 1886, p. 486. 



