TRANSACTIONS OF SECTION A. 



osr 



iron which virtually formed an extension of the conical ends of the bobbin. 

 Between these the bobbin was placed, the form generaUy used being that described 

 in the previous paper as sample A. The neck of this sample had originally a 

 diameter of 0-923 cm., and consequently a section of 0-669 square cm., or about 

 3- that of the pole-pieces. As in the former experiments, the highest values of 

 magnetism have been reached with Lowmoor iron, and nearly as high values with 

 Swedish iron. , * ^ , j -4. 



The same Lowmoor bobbin tbat had been formerly used (sample A) had its 

 magnetism measured by withdrawing it from the tield, while the magnet with all 

 its numerous coils in series was excited by a current which ranged up to 40 

 amperes. At the highest value the induction 23 in the neck was 38,000 c.g.s., 

 and the outside field, close to the neck, was 18,900 c.g.s. A Swedish sample of 

 the same shape gave an induction of 37,620 with a field of about the same force. 



To push the induction to still higher values the Lowmoor sample was then 

 turned down in the central neck until the diameter was reduced to 0-397 cm. 

 This made its section only yL of the section of the pole-pieces. Careful determi- 

 nations, several times repeated, then gave for the highest induction the enormous 

 value 43,-500 c.g.s., the outside field being 25,620 c.g.s. Here, as in the other 

 figures already given, the induction stated is that which is found after the air- 

 space enclosed by the inner coil is allowed for, and after a suitable allowance is 

 made for the residual magnetism of the piece. The residual induction is only about 

 500 c.g.s. units. 



The ratio ^ in this extreme case is 1-7, and the quantity 



outside tield 

 83 -outside field^ ^^^^^ ^^^^^^j ^^ ^^^ intensity of magnetism 3 if the magnetic 



4.77 



force within the metal were identical in value with the outside field, is 1420. In 

 the former experiments this quantity had values which decreased from 1680 to 

 1620, while the mduction increased from about 25,000 to 32,000 ; here, with an 

 induction of 43,500, it has fallen ott' to a much more marked extent. We cannot 

 yet speak with any certainty as to the degree of approximation of this quantity to 

 the intensity of magnetism 3 ; unless, however, the mean magnetic force within 

 the neck is much less than the force at the surface, the results show that 3 is 

 becoming less as the induction is being forced to these extreme values — in other 

 words, that the iron is tending towards diamagnetism in the rnanner Weber's 

 theory leads us to expect. The question is one of the greatest interest, and we 

 are now endeavouring to obtain a better knowledge of the magnetic force within 

 the metal by examining the variation of the force at short distances from the 

 surface of the neck. 



A final effort was made to force the induction in Lowrmoor iron to higher values,, 

 by turning the central neck down still further, until its section was less than j^f-^ 

 of the seclion of the pole-pieces, and annealing the bobbin carefully before^ mag- 

 netising it. The value of SB then reached was 45,350 c.g.s. units, which is the 

 greatest induction recorded in anv of our experiments. 



With cast iron the induction has been forced to 31,270 c.g.s. by applying a 

 magnetic force of 16,900. 



13. On the Magnetisation of Hadfield's Manganese Steel in Strong Fields. 

 By Professor J. A. Ewing, B.Sc, F.E.S., and William Low. 



Messrs. Hadfield of Sheffield manufacture a steel containing about 12 per cent. 

 of manganese and 0-8 per cent, of carbon which possesses many remarkable quali- 

 ties. Prominent amongst these, as the experiments of Hopkiuson, Bottomley, and 

 Barrett have shown, is a singular absence of magnetic susceptibility. Hopkinson, 

 by applying a magnetic f'rce -|) of 244 c.g.s. units to a specimen of this metal, pro- 

 duced a magnetic induction SB of only 310 c.g.s. units : in other words, the perme- 

 ability IX was 1-27, and the intensity of magnetisation 3 was a little over five units. 

 We are indebted to him for the suggestion that it would be interesting to apply to 

 this metal the ' isthmus ' method of magnetisation (the results of which, as applied 



