1882.] Stress and Strain on the Action of Physical Forces. 281 



treatment, are similar as regards their influence on the electrical 

 resistance. 



In order to make certain small corrections rendered necessary by 

 the changes of density of the metals after they had been subjected 

 to extension, hammering, or torsion, these changes were very carefully 

 measured, and were found to be in every case small. 



The amount of recovery of electrical conductivity produced by time 

 in wires, which are in a state of strain, is shown in the paper for 

 several metals by a series of curves, and these exhibit most con- 

 clusively the superiority of platinum-silver over German-silver when 

 an accurate copy of a standard resistance has to be kept for a long- 

 period of time ; in fact, of all the metals tested, Grerman- silver showed 

 the most marked recovery of conductivity, and platinum-silver the 

 least. 



The recovery of electrical conductivity is in all cases attended with 

 recovery of longitudinal elasticity and of torsional rigidity. 



A full examination of the influence of permanent strain on the 

 susceptibility to temporary change of resistance from change of tem- 

 perature showed that metals may be divided into two classes. In the 

 first of these classes, which includes iron, zinc, and platinum-silver, 

 the strained wire is most increased in resistance by rise of temperature 

 up to a certain limit of strain, whilst beyond this limit further strain 

 diminishes the first effect. In the second class, which comprises 

 copper, silver, platinum, and Grerman- silver, the strained wire is least 

 increased in resistance by rise of temperature, but that, here again, 

 after a certain point of strain has been reached, the first effect begins 

 to be diminished. It will further be shown in Part TV, that there 

 must be some close relationship between the thermo-electrical pro- 

 perties of strained and unstrained metals and their susceptibility to 

 change of resistance from change of temperature, and that strain of 

 any kind, whether produced by purely mechanical means, such as 

 traction, hammering, and torsion, or by the process of tempering, 

 renders a piece of metal thermo-electrically positive or negative to a 

 similar piece of metal unstrained, according as the strained piece is 

 caused to be less or more increased in electrical resistance by rise of 

 temperature. 



After some trouble, means were found of measuring with consider- 

 able accuracy at 100° C. the alteration of electrical resistance due to 

 temporary longitudinal traction, and the experiments led to the belief 

 that the elasticity of iron and steel is not temporarily but permanently 

 increased by raising the temperature to 100° C. Subsequently direct 

 observations of the elasticity made in the manner described in Part I, 

 but on shorter lengths of wire, placed in an air-chamber, the tempera- 

 ture of which could be maintained constantly at 100° C, proved 

 beyond a doubt that if M. Wertheim, to whom we owe so much of our 



