Currents in Wires subjected to Mechanical Strain. 341 



difference that in iron and steel wires he got a current from 

 strained to unstrained across the hot junction, while in copper 

 wire the current was from unstrained to strained across the 

 hot junction. On comparing the descriptions of the experi- 

 ments, it appeared that Thomson had always made his expe- 

 riments with comparatively small strains, while Le Roux had 

 strained his wires very nearly to the breaking limit. This at 

 once suggested a possible explanation of the apparent discre- 

 pancy between their results ; and on making the experiments, 

 it was found that as the strain was gradually increased the 

 current was increased, as in Thomson's experiments, but only 

 up to a certain limit. When the strain was increased beyond 

 this limit there was a gradual decrease in the current; and if 

 the strain was very carefully increased, the direction of the 

 current was reversed shortly before the breaking strain was 

 reached. 



During the experiments, it was very soon observed that after 

 a weight had been added the current did not remain constant, 

 but gradually diminished ; while at the same time there were 

 variations of small period in the strength of the current, which 

 were greater when the weight was added suddenly, and scarcely 

 perceptible when it was very carefully and slowly added by 

 pouring in shot; these variations gradually ceased when the 

 apparatus was not disturbed. A very gentle and gradual ad- 

 dition of weight diminished these variations, which always died 

 away more rapidly when there was a heavy strain on the wire. 

 Clutching the wire in the " dogs " also set up these variations, 

 which were allowed to subside before beginning the experiment. 

 These results suggested that the production of the current 

 might be due to a process of change in the molecular state of 

 the wire ; it was found, however, on examination that there 

 was a permanent effect which could not, as far as I can see, 

 be produced in that way, whatever may have caused the tem- 

 porary effect. 



Where the results obtained at different times had not to be 

 compared, the current is generally given in terms of the de- 

 flections of the galvanometer ; but where such comparison was 

 necessary, the ^alue of the deflections was determined at each 

 experiment in terms of a standard current obtained by send- 

 ing a current from a Daniell's cell through a definite resist- 

 ance. 



For the sake of brevity, U.S. will be written for " from un- 

 strained to strained across the hot junction;" and the oppo- 

 site direction of the current will be denoted by S.U. The 

 following letters are used in the description of the experi- 

 ments : — 



