3876.] 



to the Passage of an Electric Current. 



453 



of 1 gramme on a centimetre of the material by the increase of length 

 produced by the stretching force, so as to obtain the increase per unit of 

 resistance when the wires are stretched 1 centimetre, a mean value of 

 3-525 was obtained for the steel wires, 3*951 for the iron wires, and 

 2*203 for the brass wires — thus showing that, though the increase 

 per unit of resistance for a given stretching force is greater in brass 

 than in iron or steel, the increase per unit of resistance for a given 

 lengthening of the wire is much greater both in iron and steel than in 

 brass. 



The torsional rigidity of the wires was next ascertained by the method 

 of vibrations, several trials being made with different lengths of each 

 wire ; the results for different lengths of the same wire agreed very 

 closely indeed. 



From the values of torsional rigidity and the increase of length, the 

 diminution of section was calculated for a cubic centimetre of each wire 

 when stretched with a force of 1 gramme, assuming the wire to be 

 isotropic. Next the increase of resistance which would result from mere 

 lengthening of each wire and diminution of section was determined, and 

 it was ascertained that, on subtracting this latter value from the total 

 observed increase of resistance, there was a considerable residue in 

 the case of the steel and iron wire, also a residue not so great in the 

 brass. This residual increase of resistance probably arises from increased 

 space in the line of flow of the current between the particles of the wire 

 produced by the stretching force. 



The conclusions to be drawn from the experiments are : — 



1. That the temporary increase per cent, of resistance of a wire when 

 stretched in the same direction as the line of flow of the current is 

 exactly proportional to the stretching force. 



2. That the increase per cent, of resistance, when a cube of each 

 material is stretched by the same weight, is greater in iron than in steel 

 wire, and greater in brass than in iron ; also that the increase is nearly 

 the same for different specimens of the same material. 



3. That the increase per cent., when a cube of each material is 

 stretched to the same extent, is much greater in iron and steel than in 

 brass. 



4. That there is a residual increase in each case over and above that 

 which would follow from mere increase of length and diminution of 

 section; that this residual increase is much greater in iron and steel 

 than in brass, and greater in iron than in steel. 



