of Electricity through Metals. 197 



Thus the current is proportional to X and Ohm's law 

 holds ; the specific resistance a is given by 

 _ w-T$WkF'(0) 

 a ~ epd®M?h v (0) ' 

 Now, except at very low temperatures, w is equal to R#, so 

 that o- is expressed by an equation of the form 



It is thus a linear function of the temperature, this is very 

 approximately true for pure metals. 



Super Conductivity. 



Let us now consider what happens when the temperature 

 is diminished, the slope of the line (1) continually decreases 

 and the intersection of this, lino with the curve gets further 

 and further away from the origin; when the intersection 

 comes on a part of the curve at an appreciable distance from 

 the tangent at the origin, Ohm's law will no longer hold. 

 Suppose that the slope of the line (1) has fallen so that, as 

 in fig. 3, it is less than that of the tangent a t the origin to 



Fijr. 3. 



the curve I = NMF(«r), and after the application of a force 

 X suppose the force is gradually removed, the value of I 

 corresponding to the diminished force will be got by drawing- 

 parallels to PQ continually getting nearer to the origin, and 

 its value when the force has been entirely removed by 

 drawing a parallel through the origin itself. "We see from 

 the figure that in this case the line through the origin will 



