J8'87.] Stress and Strain oh the Properties of Matter. 225 



the metals,* which effects are for the most part opposite in nature. The 

 paper is illustrated by a large number of curves showing the relations 

 between magnetic permeability and stress and strain at different tem- 

 peratures from 0° C. to 300° C. and upwards. The information con- 

 veyed by these curves it is impossible to adequately represent in an 

 abstract, but the following are among the chief conclusions arrived 

 at : — 



1. When there is no permanent load on an iron wire, and the mag- 

 netising force is small, longitudinal traction of small amount increases 

 the temporary permeability. The increase reaches a maximum very 

 •quickly, as the load increases when further loading begins to diminish 

 tthe magnetic permeability, until a certain limit has been reached, for 

 which the permeability is a minimum. If the load be carried beyond 

 the above limit the permeability begins to increase again with the 



• load. As a consequence of the above, when the magnetising force is 

 small there are two critical values of load for which the load produces 

 no effect on the temporary permeability. 



2. The first of the two critical values of loading mentioned above 

 diminishes with increase of magnetising force, and finally vanishes 

 when the latter reaches a certain limit. 



On the contrary, the second critical value of loading increases with 

 the magnetising force. 



3. The maximum of temporary permeability mentioned in 1 di- 

 minishes as the magnetising force increases, and occurs at a less and 

 less degree of loading until the latter begins to produce decrease 

 instead of increase of permeability. 



The minimum of temporary magnetic permeability, on the con- 

 trary, increases with the ma-gnetising force, but,. like the maximum, 

 occurs with a lower amount of load the higher the magnetising 

 . force. 



4. The effects mentioned in 1, 2, and 3 as being produced by load- 

 ing, are modified when a comparatively small load is left permanently 

 on the wire. The modifications are stated in 5, 6, and 7. 



5. For small magnetising forces loading produces no effect on the 

 temporary magnetic permeability, ' unless carried beyond a certain 

 limit. Beyond this limit further loading suddenly begins to increase 



! the permeability. 



6. For all values of the magnetising force the first critical value of 

 loading vanishes. 



The second critical value of loading increases with the magnetising 

 force, but for a given magnetising force is much lower than when 

 there is no permanent load. 



* By the terms permanent and temporary permeabilities are meant tbe permea- 

 bility for permanent magnetisation, and the permeability for temporary magnetisa- 

 tion respectively. 



VOL. XLII. R 



