Barrett — On the Physical Properties of Manganese Steel. 369 



156 c- 5, or a mean of 156° for the iron, a ratio of 1 : 2'8 The 

 volumes of the bodies being as 1 : 115, the former experiment 

 gives a ratio of the susceptibilities as 1 : 345 and the latter as 

 L : 322. Hence ice may say that the manganese steel has about 330 

 times less magnetic susceptibility than soft iron ; or, if iron be 

 100,000, manganese steel will be about 300 — a very different 

 number, it will be observed, from that obtained by Mr. Bottomley, 

 for the intensity of permanent magnetisation of the two bodies. 



A few experiments were now made to determine whether this 

 method showed that the magnetic moment was directly propor- 

 tional to the volume of the material. Half-a-dozen pieces of fine 

 man ganese steel wire were cut, of equal lengths, and the upsetting 

 force and corresponding angle determined in each case. The angle 

 being 60°, the upsetting forces, less this angle, were as follows :— 



Dividing the upsetting force by the number of pieces in each 

 case, we obtain the force per piece, as shown in the last column. 

 With the exception of the second experiment — which is evidently 

 erroneous — it will be seen that the forces are directly proportional 

 to the number of pieces, and hence to the volume of the body. 



Since I began this investigation, Dr. J. Hopkinson has sent 

 me his paper on the " Magnetisation of Iron," read before the 

 Hoyal Society of London, in April, 1885. Until I received this 

 paper I was unaware that Dr. Hopkinson had been experimenting on 

 the magnetic properties of manganese steel. The method adopted 

 by Dr. Hopkinson to determine the magnetic susceptibility was 

 wholly different from that which I employed, and consisted in 

 measuring the induced current generated by the sudden removal 

 of a small coil of wire that encircled the iron or steel bar under 

 experiment, and which bar had previously been submitted to a 

 powerful magnetising current. The maximum magnetisation of 



SCIEN. PROC. R.D.S. VOL. V. PT. V. 



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