264 THE ELECTRIC TELEGRAPH. 



occurs, however, that the value of x comes out greater than 

 JS", which is no absurdity ; as it proves only that then N is not 

 great enough to give us the maximum which x expresses, and 

 in this case we must take all the elements in series. 



III. CONDUCTING POWERS or MATERIALS. 



29. Specific Conducting Powers. The conducting power of 

 a material is independent of the form and dimensions of the 

 body measured. AVe have already seen that the resistance of 

 a geometrical body of any material is directly proportional 

 to its length and inversely to its sectional area ; it is also 

 inversely proportional to its conducting power. By length 

 is understood the distance between the points where a current 

 enters and where it leaves the body ; by sectional area, the 

 section at right angles to the direction of the current through 

 the body, or to the line joining these two points ; and by its 

 conducting power, the ability which the material has to 

 communicate the electricity from atom to atom along its 

 length. 



Algebraically expressed, therefore, the resistance r of any 

 body is 



I 



80 



I being its length, s its section, and c its conducting power. 

 From this we have the value of <?, 



= 



There is no absolute measure of conducting power any 

 more than there is of specific gravity; and it becomes in 

 consequence, necessary to refer the conducting powers of all 

 materials to that of some one as unit, just as we refer the 

 specific gravities of bodies to that of water. Physicists are 

 not quite agreed what they shall take as unit of conducting 

 power. Lenz, Siemens, and others have adopted pure mercury, 

 and Matthiessen has advocated an alloy of gold and silver, or 

 pure silver. 



