588 EEPOET— 1887. 



to wrought iron and cast iron, have been described in a former paper), with the 

 ■view of seeing whether the magnetic resistance of manganese steel could be broken 

 down by applying a very strong magnetising force. 



Messrs. Hadtield were kind enough to supply a sample of the metal for experi- 

 ment, out of which a bobbin was turned, with some difficulty, of a form resembling 

 those used in testing wrought iron and cast iron, but with a wider central neck. 

 The bobbin was magnetised by placing it between the pole-pieces of Professor 

 Tait's large magnet ; and the induction within the neck, and also the field in the 

 air immediately surrounding the neck, were measured in the usual way, by the 

 help of two induction coils and by drawing the bobbin suddenly out from between 

 the magnet-poles. A large number of readings were taken, while the field magnet 

 was excited with currents ranging from about 1 to 40 amperes. These gave values 

 of the magnetic field (in the air immediately surrounding the central neck of the 

 steel bobbin) ranging up to 5,200 c.g.s. units, and values of the induction 23 within 

 the neck ranging up to 7,700 c.g.s. units. To be more exact, these latter were the 

 values of that part of the induction which disappeared when the metal was drawn 

 out of the field, but the correction for residual magnetism was probably negligible. 

 The ratio of induction to field had a nearly constant value when the field ranged 

 from about 1,000 c.g.s. to 5,200 c.g.s. ; the values of this ratio calculated from the 

 observations fluctuate somewhat, but do not appear to undergo any progressive 



change. The mean value of —, — r^r-, is 1'45, a quantity which we may prob- 



■^ outside field ' "i J J ^ 



ably take without substantial eiTor as the value of the permeability /i. 



To test the influence of still stronger magnetic fields a second series of experi- 

 ments was made with a composite bobbin made up of a cylindrical shank — extend- 

 ing from end to end — of manganese steel, and conical pole-pieces of soft wrought 

 iron forced on to the steel shank so as to leave only a short length of it (about 

 three millimeters) bare in the middle. With a given current in the field magnets 

 this gave much higher values of the field and the induction in the central neck, 

 because the wrought iron cones now substituted for the conical ends of the solid 

 steel bobbin that had been formerly used gave an easier path for the lines of induc- 

 tion to converge to the central neck. The field now ranged up to values sliglitly 

 exceeding 10,000 c.g.s. units, and the highest induction reached was about 15,000 

 c.g.s. units. In this series of experiments, as in the former series, the ratio of 

 induction to field fluctuated irregularly ; but its mean value was nearly identical 

 with the former mean — namely, 1'40. The intensity of magnetisation 3 was forced 

 up to values lying between 300 and 400 c.g.s. 



The experiments make it clear that, even under magnetic forces extending to 

 10,000 c.g.s. units, the resistance which this manganese steel offers to being mag- 

 netised suffers no breakdown in any way comparable to that which occurs in 

 wrought iron, cast iron, or ordinary steel at a very early stage in the process of 

 magnetisation. On the contrary, the permeability is approximately constant under 

 large and small forces. This conclusion has some practical interest. It has been 

 suggested that this .steel should be used for the bed-plates of dynamos, and in other 

 situations where a metal is wanted tnat will not divert the lines of induction from 

 neighbouring spaces. In such cases the magnetic forces to which manganese steel 

 would be subjected would certaiulj' lie below the limit to which the force has been 

 raised in these experiments. We may therefore conclude that in these uses of the 

 material it may be counted upon to exhibit a magnetic permeability only fraction- 

 ally greater than that of copper, or brass, or air. 



SATURDAY, SEPTEMBER 3. 

 The following Reports and Papers were read : — 

 1. Second Heport of the Committee on Electrolysis. — See Reports, p. 336. 



