Stress and Strain on the Properties of Matter. 531 



stress per square unit of area and per unit temporary increase of 

 length, is much greater than can be accounted for by changes in the 

 dimensions of the steel. 



8. The electrical resistance of magnesium is temporarily increased 

 by temporary longitudinal traction of moderate amount, but the 

 amount of increase is less than can be accounted for by mere change 

 of dimensions, so that the specific resistance of magnesium, like that 

 of aluminium, is diminished by the temporary stress. 



9. When the permanent load on the wire is very small, the temporary 

 increase of length, like that of the increase of resistance, increases in 

 larger proportion than the temporary load, but the former increases 

 less rapidly than the latter, so that when the temporary stress exceeds 

 a certain limit the above-mentioned decrease of specific resistance is 

 changed to an increase of specific resistance. 



10. The values of "Young's modulus," and the simple rigidity of 

 the alloy, platinum-iridium, are much greater than those calculated 

 from the same values for the components of the alloy. 



11. The electrical resistance of platinum-iridium, quite unlike that 

 of platinum-silver, German-silver, and brass, is much more increased 

 by temporary longitudinal traction than that of either of the com- 

 ponents of the alloy. 



12. The increase of resistance mentioned above is much greater 

 than can be accounted for by change of dimensions, so that the 

 increase of specific resistance produced by longitudinal traction is 

 considerably greater than is the case with any of those other metals 

 examined whose resistance is increased by longitudinal traction. 



13. The alteration of the resistance alluded to in 11 at first increases 

 in greater proportion than the load, but when a certain limit of stress 

 has been reached, the ratio of the temporary increase of resistance to 

 the load producing it, begins to diminish, and finally reaches the same 

 value as at first. A tendency to a similar state of things is seen with 

 other metals, but in none is the phenomenon so pronounced as in 

 platinum-iridium. 



14. Unstretched platinum-iridium is thermo- electrically positive to 

 the temporarily stretched metal. 



15. The present investigations, as far as magnesium is concerned, 

 confirm the previous ones in showing an apparent relationship between 

 the " Hall effect " and the alteration of the specific resistance pro- 

 duced by mechanical stress. With regard to cobalt this is not so, at 

 any rate for the metal when not under the influence of magnetic 

 stress. 



