CONDUCTION OF ELECTRICITY IN MOLYBDENITE. 



223 



then in the other, and in measuring in each case the four 

 potential differences A — B, B-O, O-D, and A - D, the 

 ratio of the potential difference between any two of these 

 points to that between the terminals of the known resist- 

 ance giving a comparative value of the apparent resistance 

 between the two points. As the contact resistances are 

 at A and D, in the case of B — O this ratio should depend 

 only on the dimensions of the molybdenite strip between 

 these points and on the resistivity of the material; if the 

 latter is constant the ratio is independent of the value of 

 the current. In the case of the other pairs of contacts the 

 ratio depends also on the value of the contact resistance 

 at A or D or both. 



In figure 2, curves are given for a specimen tested at 

 17° C; the results shown there are typical. In these curves 

 the abscissas are proportional to currents, the values in 



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Fig. 2. 



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