PIERCE. — CRYSTAL RECTIFIERS FOR ELECTRIC CURRENTS. 345 



The resistances of these two conductors were measured at various 

 temperatures with the aid of a Wheatstone bridge. They showed no 

 evidence of rectification. In making the measurements it was necessary 

 to keep the current small so as to avoid electrical heating of the con- 

 ductors. With successive heatings and coolings the resistance of the 

 molybdenite showed small progressive changes, which, however, after 

 some months almost disappeared. When the resistance of the two 

 specimens of molybdenite had settled down to a practically steady con- 

 dition, the values plotted in Figure 11 were obtained. The curves 

 marked " 50 " and " 51 " give the resistances of No. 50 and No. 51 re- 

 spectively. The ordinates for these curves are at the left margin of 

 the diagram, and are in ohms. The curves " C 50 " and "C 51 " are 

 for the reciprocals of the resistance of No. 50 and No. 51 respectively. 

 The ordinates for these curves are at the right-hand margin of the 

 diagram. 



Each of the specimens has a large negative temperature coefficient 

 of resistance. With No. 50, for example, the resistance at 93.1° C. is 

 229 ohms; at 0° C. the resistance is 561 ohms ; at — 76° the resistance is 

 3051 ohms ; and at the temperature of liquid air the resistance of this 

 specimen was found to be over 6,000,000 ohms. This last value is not 

 plotted on the curves. 



It is interesting to note that between —15° and 93° the temperature- 

 conductance curve of each of the specimens is a straight line. 



At 0° C. the resistance of each of the specimens decreases about 1.53 

 per cent per degree centigrade increase of temperature ; at 20° the 

 decrease of resistance per degree increase of temperature is 1.19 per 

 cent. 



A previous determination of the resistance of molybdenite has been 

 made by Otto Reichenheim. 15 He did not solder on his connections, 

 but led the current into the specimen through contact electrodes and 

 found that the resistance depended on the contact pressure. His data 

 are, therefore, not comparable with mine, but I find that one of his 

 specimens, 16 measured parallel to the direction of cleavage, gives the 

 conductance a linear function of the temperature between 19.5° and 

 92.5° C, with a slope not very different from that obtained in the 

 present experiments. 



The large thermoelectromotive force of the molybdenite against the 

 common metals, together with its large negative temperature coefficient 

 of resistance, lends plausibility to the hypothesis that the rectification 



18 Otto Reichenheim, Inaugural Dissertation, Freiburg, 1906. 

 16 Described as Stab II, p. 27 of the Dissertation. 



