334 Dr. E. van Aubel's Researches on 



Afterwards the resistance varies, between the same limits, 

 according to the formula :— 



r t =r (1 + 0-00375* + 000008* 2 ), 

 and the mean coefficient of variation of the resistance between 

 0° and 100° is then 0*00455. 



For a thin plate of bismuth, Leduc has found that after 

 annealing the resistance decreases, between 0° and 70°, 

 according to the following formula : — 



n=r (l -0-00158* + 0-00000437* 2 ). 

 And this gives the number —0*00127 for the coefficient of 

 variation with the temperature between 0° and 70°. The re- 

 sistance of the metal thus prepared would be much greater 

 than that given by Matthiessen, perhaps more than double 

 the value found by him, no doubt because of the difference of 

 molecular structure of the specimens examined. 



In 1884, Prof. Righi * published a very remarkable paper, 

 and we will sum up those of his conclusions which bear 

 directly upon our researches. 



1. The resistance of commercial bismuth increases on heat- 

 ing between certain limits of temperature, and decreases 

 between others (generally it decreases at temperatures near 

 the ordinary temperature), and, by constructing a curve of 

 resistance, with the temperatures as abscissae, and the specific 

 resistances as ordinates, we obtain in general a curve in the 

 form of an M. There is a maximum at a low temperature, then 

 a minimum, then a second maximum a little below fusing 

 point, and finally a second minimum after the change of state. 



2. The resistance of commercial bismuth varies not only 

 with the temperature, but also with the manner in which the 

 piece has been prepared, and with the temper of the metal. 



3. Chemically pure bismuth behaves like other metals ; it 

 is not sensibly affected by tempering, and at 0° its resistance 

 compared with that of mercury is 1*15. 



4. The difference between pure and commercial bismuth is 

 due to traces of tin, which give to the Litter properties 

 similar to those by which steel differs from iron. 



5. On adding to bismuth tin in increasing amounts, the 

 specific resistance becomes much greater up to a maximum, 

 and then diminishes. 



6. The presence of tin in increasing amounts modifies the 

 curve of resistance, in the same manner as a temper more 

 and more hard would do. 



G. Wiedemann t thinks that the phenomena observed 



* Journal de Physique, [2] iii. (1884) p. 355. 

 t Elektricitat, Bd. iv., ii. ; p. 1228. 



