Stress and Strain on the Properties of Matter. 



525 



whereas here the increase of specific resistance produced by traction 

 is nearly three times as great as with iron, which heads the list of 

 those metals which show increase of resistance on loading. 



Experiment XI. 



Under the above-mentioned circumstances, it was deemed to be 

 advisable to test the platinum-iridium in another manner. In this 

 case, instead of using German-silver as the comparison- wire, the 

 following arrangement was made : — 



To the centre of the wire was firmly fixed a brass clamp, which 

 rested on a stout table, and was connected with one terminal of the 

 galvanometer, the other terminal being as usual connected with the 

 sliding-piece. The lower half of the wire, which passed through a 

 small hole in the table, was stretched by means of the lever, whilst 

 the upper half remained unstretched. Near the extremities of both 

 halves were fixed other clamps, provided with terminal screws, which 

 in each case were united as usual with one pole of a battery, and with 

 a set of resistance coils joined to each other by the platinum-iridium 

 wire traversed by the sliding-piece. With this arrangement it will be 

 seen that the upper half of the wire served as a comparison- wire to 

 the lower half. The results of this experiment were in fair accord- 

 ance with those of the previous one, and still showed platinum- 

 iridium to be considerably more increased in resistance by traction 

 than any of the other metals. 



Experiment XII. 



In order to remove any further doubt about clamping not being 

 sufficient to make proper connexion between the various parts of the 

 "bridge," a third experiment was tried, in which the platinum- 

 iridium wire, with a German-silver wire as a comparison- wire, were 

 arranged as in Experiment X, but now all connexions with the wire 

 to be stretched were well soldered, and a long series of trials, extend- 

 ing over three days, was made with a view to not only confirm the 

 results of Experiments X and XI, but also to bring out any fresh 

 peculiarities which might exist. The main points to which it is well 

 to call attention are shown in the last two series of trials given below, 

 in which the permanent stress was that due to the weight of the 

 lever itself. 



It is noteworthy here that the resistance at first increases in greater 

 proportion than the load, but when a certain limit of stress has been 

 reached the ratio of the increase of resistance to the load producing it 

 begins to diminish, until finally the last kilogram only produces the 

 same alteration of resistance as the first. A similar state of things 

 was perceived with wires made of other metals in the experiments 



