PIERCE. — CRYSTAL RECTIFIERS FOR ELECTRIC CURRENTS. 341 



corks. Each cork, 4.5 cm. long, was split lengthwise, and one of the 

 longitudinal half-corks was grooved out to contain the molybdenite. 

 The two half-corks with the molybden- 

 ite between were put together again 

 and cemented with plaster of Paris, so 

 as to leave 2 or 3 mm. of molybdenite 

 protruding from each end of the cork. 

 These small areas were then copper- 

 plated, and copper wires .2 mm. thick 

 were soldered to the copper-plated 

 areas, so as to form thermal junctions. 

 The four corks containing the speci- 

 mens A, B, C, and D were inserted in 

 round holes in two copper vessels for 

 containing the temperature baths of 

 oil, so that the junction at one end of 

 each specimen should be in the hot 

 bath, while the junction at the other 

 end was in the cold bath. The cold 

 bath was kept at 20° C. ; the hot bath Figure 10. Thermoelectromotive 

 was given various temperatures be- force , °* A^ copper-molybdenite 



^^ i *™o n mi , 1 couples, for various temperatures of 



tween 20 and 100° C. The thermo- hot junct ion. Temperature of cold 

 electromotive force of each couple was junction, 20° C. 

 measured on a potentiometer. The 



results for A, B, C, and D are contained in Table IX and are plotted in 

 Figure 10. For comparison a part of the curve obtained for E is also 

 plotted in Figure 10. 



Some of the specimens (B, D, and E) are thermoelectrically negative 

 with respect to copper, while the other specimens (A and C) are ther- 

 moelectrically positive with respect to copper. The thermoelectro- 

 motive force per degree differs largely with the different specimens, as 

 may be seen by a reference to Table X, which contains the thermo- 

 electromotive force per degree of the different specimens of molybdenite 

 against copper and against lead (obtained from the known value of the 

 lead-copper junction). For comparison Table X also gives the thermo- 

 electromotive power of some other remarkable thermoelectric elements. 



The comparison shows that these specimens of molybdenite have 

 very large thermoelectromotive force against copper or against lead. 

 The specimens D and E were found to be at the extreme negative end 

 of the thermoelectric series. 



The great variability among the specimens studied may be due to 

 an admixture of small quantities of some other substance with the 



