PIERCE. — CRYSTAL RECTIFIERS FOR ELECTRIC CURRENTS. 343 



patches in a piece of the molybdenite may indicate that the thermo- 

 electromotive force measured by attaching wires to the specimen is too 

 low on account of the inclusion under the electrodes of both positive 

 and negative areas which would partially neutralize the thermoelectric 

 action against another electrode. 



TABLE X. 



It may be said in passing that the specimens D and E, with the 

 soldered connections, still showed the phenomenon of rectification when 

 used with alternating currents, even when the two junctions of the 

 copper with the molybdenite were in oil baths at the same temperature 

 as the room and the oil in the baths was vigorously stirred with motor- 

 driven stirrers. The rectification in this case was, however, very im- 

 perfect. 



Temperature Coefficient of Resistance. — Another interesting thermal 

 property of the molybdenite is its temperature coefficient of resistance. 

 A preliminary report of this coefficient is here given. 



