268 THE ELECTRIC TELEGRAPH. 



ment between the values found and calculated is obtained 

 when the constant y is not neglected, as in these calcula- 

 tions, but the member containing it (y t 3 ) introduced, by 

 which the coincidence of the found with the calculated curve 

 extends far beyond the limits of C. and 100 C. But 

 within these limits it is near enough to take the values of 

 the constants a, j3> y> &c., alternately positive and negative, 

 and to regard those beyond |3 as = 0. 



Hence the resistance, B^ of a pure metal wire, at the 

 temperature t, whose resistance at C. is R , is 



__ E / 



" ^ 



100 



1000,37647 t + 0,000834 



Two exceptions are found to this rule, in the metals iron 

 and thallium : the per-centage variation of the conducting 

 power of pure iron between and 100 is 39 '2, and that of 

 pure thallium between the same limits, 31*4, while the other 

 pure metals vary only 29-3 per cent. 



32. Alloys. The conducting powers of alloys of lead, 

 tin, cadmium, zinc, and some other metals, with each other, 

 are proportional to the volumes of the metals entering into 

 their compositions. These metals form a class separate from 

 the others, and are of limited number. For the most part 

 the metals belong to the other class, those which, when 

 alloyed with each other or with one of the metals above 

 mentioned, have smaller conducting powers than are propor- 

 tional to their respective volumes. To this class belong 

 bismuth, antimony, platinum, palladium, iron, aluminium, 

 sodium, gold, copper, silver, and so on 



This difference is the result of the different natures of the 

 alloys, and depends upon the nature of "the combination of 

 the metals forming them. Many alloys are unquestionably 

 chemical combinations, others are solutions of one metal in 

 another, others, perhaps, only mechanical moistures, and 

 others, again, solutions of one of these in an excess of one 

 of the metals. 



Of the alloys which enter most largely into matters con- 

 nected with telegraphy, German-silver deserves to be espe- 



