Barrett — On the Physical Properties of Manganese Steel. 367 



mirror magnetometer. The number so obtained showed a magnetic 

 moment fx = 2-55 C. Gr. S. units. Dividing this by the weight of 

 the steel we obtain the magnetisation per gramme, which is 0*013 

 C. Gr. S. units. Ordinary steel has a number ranging from 40 to 

 60, and even up to 100, O. Gr. S. units per gramme. 



Hence the ratio of the intensity of magnetisation in manganese 

 steel to that in ordinary steel is as 1 to 3000, up to 1 to 7700 in the 

 best qualities. 



So that the intensity of magnetisation that can be given to 

 manganese steel is, say, 5000 times less than that given to steel of 

 average quality; or if steel be 100,000, manganese steel is 20. 



This refers to the degree of permanent magnetism that can be 

 imparted. It is important to know the co- efficient of induced 

 magnetism of this remarkable body. This co-efficient, designated by 

 K, is the ratio of the intensity of induced magnetisation to the 



magnetising force of the field, or K = — : this is now generally 



termed the magnetic susceptibility of the substance. 



The experimental determination of this constant, for so feeble 

 a, magnetic body as manganese steel, proved a more difficult task 

 than I anticipated, as it is scarcely comparable with iron, and 

 therefore like weighing stones and grains on the same balance. 



I first tried the method of torsion adopted by Faraday in the 

 determination of the magnetic force of magnecrystallic bodies, and 

 described by him in the last of his " Experimental Researches in 

 Electricity," Phil. Trans., 1855. 



A platinum wire, hung from a graduated torsion head, sus- 

 pended the specimen under examination in a powerful and uniform 

 magnetic field obtained from a large electro-magnet. A graduated 

 circle was placed below the object under trial, the zero coinciding 

 with the axial position of the object. On exciting the magnet, 

 and then turning the torsion head, the object was twisted out of 

 its axial position, and at last reached a position of unstable equili- 

 brium, when it suddenly swung round to the axial position again, 

 but with reversed ends. The degree of torsion required, minus the 

 upsetting angle, was used by Faraday to " measure the force which 

 solicits the body to retain its axial position," that is to say, it is a 

 relative measure of the magnetism induced in the body, or its 

 susceptibility. 



