Magnetic Permeability of various Alloys of Iron. 



103 



point of maximum induction, and another series down gives the part 4 to where 

 the curve cuts the axis OB again ; the circuit is again broken and the current 

 reversed, and the hist series of steps up gives the part of the curve marked 5, 5. 

 This completes the cycle, but another series of steps down was always taken, 

 —i.e. repeating the portion 2 of the curve — and finally another series of steps up, 

 corresponding to the upper part of 3, so as to leave the rod in a practically non 

 magnetic state, thus saving time if we found that its cycle had to be repeated. 

 In every case the whole cycle was gone over again, if by inspection of our 

 observations we found any discrepancy between the upper and lower halves of 

 the cycle, that is if the cyclic curve was 

 not perfectly symmetrical above and below 

 the axis OH. In the plates ajipended to 

 this paper the position of the cycle below 

 the axis of OH is therefore omitted. The 

 initial curve 1, fig. 8, is shown in all the 

 plates as a finer line than the continuous 

 cycle of the particular specimen, but repre- 

 sented in the same kind of dotted or 

 continuous curve. For convenience of 

 comparison the B and H curve obtained 

 for our standard rod of iron is reproduced 

 in all the plates. 



The residual induction of the specimen is 

 expressed by the distance OR (fig. 8) ; that 

 is to say, the induction which remains in a 

 very long thin rod after the magnetising 

 current is removed : for brevity we will 

 call this the retcntivity .* The coercive force 

 is expressed in terms of H by the distance OC ; that is to say, the magne- 

 tising force required to demagnetise the specimen. The magnetic permeability 

 of the siiecimcn varies, of course, with the magnetising force employed, and is 

 expressed by the ratio of the induction B to the magnetic field H at any given point. 

 The hysteresis loss can be deduced from the areas enclosed by the B-H curves. t 



* As the residual induction increases with the maximum induction the ratio of these two quantities is, 

 strictly speaking, the retentivity. The coercive force also rises with the maximum induction. 



t For the measurement of most of these areas we are indehted to Mr. R. L. Wills, a former senior student 

 in the Physical Laboratory of the Eoyal College of Science, Dublin, now working in the Cavendish 

 Laboratory, Cambridge, as an 1851 Science Research Scholar. We must also acknowledge the useful 

 assistance rendered to us in the laborious determination of these B and H curves by Mr. Wills, in con- 

 junction with Mr. R. G. Allen, another Research student in the Physical Laboratory of the Royal College of 

 Science, Dublin. 



Fig. 8. 



