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VI. The Effect of repeated Heating and Cooling on the Elec- 

 trical Resistance of Iron. By Hekbert Tomlinson, 

 FM.S* 



SOME of the physical properties of iron wire, even if it 

 has been previously well annealed, can be considerably 

 modified by repeatedly raising the metal to the temperature 

 of 100° C, and suffering it to cool again. The internal 

 friction, for instance, of a torsionally oscillating iron wire can 

 be largely and permanently reduced by this process. Again, 

 in a paper quite recently presented to the Royal Society, the 

 author has brought forward an instance of an iron wire, which 

 when made to go through magnetic cycles of very minute 

 range, alternately at the temperatures of 100° C. and 17° C, 

 was found to be permanently reduced, both in its molecular 

 friction and its magnetic permeability, at each heating and 

 cooling to such an extent that ultimately the former of these 

 two physical properties became one quarter, and the latter less 

 than one half of their respective original amounts. The large 

 diminution of permeability and friction was also attended by 

 a considerable lessening of the temporary effects of change of 

 temperature on these properties. The object of the present 

 investigation was to ascertain whether the electrical resistance 

 and temperature-coefficients of iron would also be altered by 

 the heating and cooling process. According to Matthiessen 

 the electrical resistance of metals can be expressed by the 

 formula 



R^=R (1 + at + bt 2 ), 



where R^ and R are the resistances at t° and 0° C. respec- 

 tively, and a and b are constants. For most pure metals the 

 coefficient a is not very far from '00366,, and the resistance 

 varies, therefore, approximately as the absolute temperature ; 

 but with pure iron this coefficient is much greater. One of the 

 questions which the inquiry was designed to answer was — Can 

 the temperature-coefficient of iron be reduced by repeated 

 heating and cooling to anything like '00366 ? This question 

 has been answered in the negative, but, at the same time, it 

 appears that the electrical resistance itself suffers a small but 

 decided change. 



The iron wire examined formed part of a hank supplied by 



* Communicated by the Physical Societv : read November 15, 1889. 



The Author begs to acknowledge, with thanks, the assistance which he 

 has received, in this and kindred investigations, from the "Elizabeth 

 Thompson Science Fund." 



