640 CONSTANTS OF MAGNETISATION. 



annealing. The coefficient of variation of the resistance with the 

 temperature is connected with the resistance itself, by a law which 

 seems to be the same for different kinds of iron, as is seen by the 

 curve of Fig. 249, in which the specific resistances have been taken 

 as abscissae, and the variation for i at zero as the ordinate, the unit 

 being the microhm. 



The thermoelectric power of steel varies with the resistance, and 

 proportionally so, according to these same physicists. The numbers 

 are sensibly constant for steel of the same origin ; they only vary 

 within narrow limits, but in the same direction, from one specimen 



o.e 



0.5 \ Soft iron 



^ ft "' 



Blue tempered steel 

 3 >v Yellow tempered steel 



0.2 ^x. Hard steel 



~ s ' , Hard 



Soft cast iron (asf {ro 



Specific resistances 



10 20 30 40 50 60 70 80 90 100 



Fig. 249. 



to another. It appeared natural, therefore, to define the condition 

 of the steel, either by its specific resistance or by its thermoelectric 

 power. 



We find, in this way, that the effect of annealing is felt markedly 

 even for temperatures below 100 a circumstance which must be 

 taken into account. For a given temperature, the effect of annealing 

 increases with the time, and tends towards a limit. 



In order to investigate the influence of the condition of the steel 

 on the permanent magnetism, experiments have been made with very 

 thin steel bais of the kind called silver steel They were raised to 

 a red heat by an electrical current, and then suddenly cooled by a 

 current of water ; they were then magnetised to saturation by placing 



