266 



THE ELECTRIC TELEGRAPH. 



30. Pure Metals. The most recent determinations are 

 those of Dr. Matthiessen, who, setting the conducting power 

 of pure silver at C. = 100, makes the conducting powers 

 of the other metals at that temperature and at 100 0. as 

 follows : 



Still more convenient is the comparison of the conducting 

 powers with that of pure mercury at C., on account of its 

 low conducting power. 



31. Influence of Temperature. The resistance which a 

 body of any material whatever offers to the passage of an 

 electric current varies when the temperature of the body is 

 changed. The conducting powers of the metals decrease as 

 their temperature is increased ; but those of the oils, gums, 

 most dielectrics, and metalloids, increase as their temperature 

 is raised. 



For this reason it is essential, in all quantitative electrical 

 measurements, to know the temperature of the body which 

 is measured as well as of that with which it is compared ; 

 which enables us, being provided with the co- efficients of 

 variation of the materials, to reduce the value found to some 



