78 Intelligence and Miscellaneous Articles. 



then, regularly as the temperature sinks ; between — 80° and 

 — 92°* 13 it is represented by the formula 



1+at 



in which t is the temperature on the centigrade-scale with 



a = 0-00407. 

 This coefficient of variation a, which is almost five times that for 

 liquid mercury, is very near that of other pure liquids in the solid 

 state. 



2. Silver, Aluminium, Magnesium, Tin, — For these various metals 

 the resistance is represented by the formula 



r t = r (l + a,t); 



and the values of a,, deduced from numerous experiments made at 

 various temperatures, are the following : — 



Metal. oc. Range of temperature. 



Silver 0-00385 +29-97 to -101-75 



Aluminium .... 0-00388 +2777 to - 90-57 



Magnesium.... 0-00390 to - 88*31 



Tin 0-00424 to - 85-08 



These values of ct, which are very near those for the same bodies 

 near 0°, according to Matthiessen's experiments, are almost identical 

 for the first three metals ; the value of a for solid mercury is 

 between the value common to magnesium, aluminium, and silver, 

 and that for tin. 



3. Copper. — The most complete experiments are those which we 

 have made with copper. They gave the following values for a 

 deduced from a series of thirty measurements, which have been 

 divided into three groups : — 



a. Kange of temperature. 



f 0-00418 6 to - 58-22 



Copper \ 0-00426 - 68-65 to -101-30 



[ 0-00424 - 113-08 to - 122S2 



These values are a little greater than those which follow from 

 the formulas of Matthiessen and of M. Benoit for temperatures 

 near 0°*. The variation of resistance is almost absolutely regular, 

 and would enable us, in case of need, to dispense with a hydrogen- 

 thermometer in measuring temperatures between —20° and —123°. 

 No appreciable variation of a is observed at this latter temperature, 

 which would seem to indicate that the agreement, at any rate ap- 

 proximate, of the hydrogen- thermometer and the copper spiral 

 thermometer might be pursued still further. 



4. Iron, Platinum. — Both these metals differ greatly from the 

 others in the variation of their resistance above zero ; they diverge 

 from it in the same direction at low temperatures. The formula 



V asf o( 1 +*9 



* a = 0-00367 (Matthiessen); 0-003637 (Benoit). 



