1882.] Stress and Strain on the Action of Physical Forces. 279 



Part I, which concern the temporary alterations of length produced 

 by longitudinal traction. 



One of the most remarkable features discernible in the table is the 

 similarity of the order of the metals, as given in the last column, to 

 that of the table of " rotational coefficients " of metals recently given 

 by Professor Hall ;* indeed, so striking is the relationship in the case 

 of the metals iron, zinc, aluminium, and nickel, that there would 

 appear to be no doubt that a series of experiments made with a view 

 of determining the effects of mechanical stress and strain on the 

 " rotational coefficients " would be of the greatest value. 



Another point to be noticed is, that the alteration of the specific 

 resistances of the alloys brass, platinum-silver, and German-silver is 

 much less than that of the several constituents of these alloys, and at 

 first sight there would appear to be some relation between the altera- 

 tion of resistance caused by change of temperature and that due to 

 mechanical stress ; but it has been proved by these and other experi- 

 ments that the increase of resistance caused by rise of temperature is 

 in some cases one hundred times that attending the same amount of 

 expansion by mechanical stress ; and, apart from the fact that with 

 nickel and carbon the effects of change of temperature and of longi- 

 tudinal stress are of an opposite nature, it is evident that the former 

 are to be attributed to other causes than mere expansion. 



The influence of permanent extension on the temporary alteration 

 of resistance caused by longitudinal stress was examined, and the 

 results obtained verified the statement made in Part I that "the 

 elasticity of a wire is diminished by permanent extension not exceeding 

 a certain limit, but beyond this limit it is increased." The effect of 

 permanent extension on the alteration of resistance which can tem- 

 porarily be produced in nickel by traction is very remarkable. 



Compression was proved to produce on the electrical resistance of 

 carbon a contrary effect to that caused by extension ; this statement 

 applies to the alteration of specific resistance as well as of the total 

 resistance. 



Stress, applied in a direction transverse to that of the current, was 

 also found to produce in several metals both temporary and permanent 

 alterations of resistance of a nature opposite to those resulting from 

 longitudinal traction. The time during which the stress was allowed 

 to act exercised with strips of tin and zinc a large influence on the 

 amount of the temporary alteration of resistance which was produced 

 by the stress. In the case of the strips of tin and zinc also, the 

 alteration of resistance seemed to be very much greater than when 

 longitudinal stress was applied to these same metals in the form of 

 wires. 



* " Nature," November 10, 1881. Abstract of a note read by Professor E. H. 

 Hall at the meeting of the British Association at York. 



