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Mr. H. Tomlinson. The Influence of [Jan. 26, 



Stress applied equally in all directions by means of an hydraulic 

 press was proved to diminish the resistance of copper and iron ; and 

 the experiments showed that the lowering of the temperature of the 

 freezing point of water can be accurately and readily measured by 

 observations of the change of electrical resistance of a wire. These 

 experiments also furnished still farther proof that the change of 

 resistance of a metal wire caused by rise of temperature is due almost 

 entirely to other causes than mere expansion. 



Experiments on the permanent alterations of resistance which can 

 be produced by stress, furnish valuable information respecting the 

 " limit of elasticity " of metals. 



There are two " critical points " in every metal at which sudden 

 changes occur in the ratio of the permanent extension due to any load 

 and the load itself. The first of these points fixes the position of the 

 true limit of elasticity, and the second that of the " breaking-point." 

 With iron there are three, and, perhaps, more " critical points." 



The total resistance of most metals is permanently increased by 

 permanent longitudinal extension, but with nickel the total resistance 

 is permanently decreased, provided the extension does not exceed a 

 certain limit : beyond this limit further extension causes the resistance 

 to increase. 



The rate at which a wire is "running down " under the influence of 

 a load can be very advantageously studied by observing the permanent 

 increase of resistance produced by the load. If P be the " breaking- 

 load " of a metal, and^> be the load actually on the wire, the decrease 

 per unit of the velocity of the increase of resistance is inversely pro- 

 portional to P — p : so that the breaking-load of a wire can be calcu- 

 lated from observations of the rate of increase of resistance when a 

 loaded wire is "running down." The above-mentioned proportion is 

 constant not only for one and the same metal, but for all metals. 



The small effects which can be produced by permanent extension, 

 hammering, and torsion on specific electrical resistance were very 

 fully investigated, and are shown in the paper by a series of curves. 

 All the metals examined, except iron and nickel, have their specific 

 resistances increased by strain caused by the above-mentioned pro- 

 cesses, provided the strain does not exceed a certain limit, beyond this 

 limit further strain decreases the specific resistance. In the case of 

 iron and nickel, on the contrary, the specific resistance is at first 

 decreased and afterwards increased. 



The effect on the resistance of annealed steel produced by heating 

 and suddenly cooling was also studied, and it was proved that if the 

 steel be heated to a temperature under "dull red," sudden cooling 

 decreases the resistance ; whereas if the metal be heated up to or beyond 

 " dull red," sudden cooling increases the resistance : the strain, there- 

 fore, caused by this process, and that resulting from purely mechanical 



