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PROCEEDINGS OF THE AMERICAN ACADEMY. 



is due to thermoelectricity. For if we pass an electric current through 

 the rectifier and the current begins to make its way through a small 

 area at the contact, this small area is heated and decreases in resistance, 

 so that the greater part of the current flows through this particular 

 small area, heating it still more, while the portions of the contact 

 through which the current has not started remain cool and continue to 

 offer a high resistance. The effect of this action is to confine the heat- 

 ing to an extremely small area, which is the condition necessary for the 

 extremely rapid and efficient action of the rectifier. That there is, 

 however, strong evidence against this explanation of the phenomenon 

 is, I think, made clear in the succeeding experiments. 



Experimental Facts Adverse to the Thermoelectric Explanation 

 or the Phenomenon of Rectification. 



Thermoelectric Effect Opposite to the Rectification. — A number of 

 experiments with different specimens of molybdenite were made, in 



which the rectification and the 

 thermoelectric effect could be sim- 

 ultaneously studied. A diagram 

 of the arrangement of apparatus is 

 given in Figure 12. The specimen 

 of molybdenite is shown at M, and 

 was held down upon a wooden base 

 by a spring clip. One end of each 

 specimen, which was easily inter- 

 changeable in the apparatus, was 

 electroplated with copper at S. To 

 this copper-plated area a copper 

 lead was soldered. A copper rod 

 C, supported as in Figure 3, was 

 brought into contact with the part 

 of the molybdenite distant from the 

 soldered junction. The molybden- 

 ite and the contact were put in an 

 electric circuit containing a microammeter or galvanometer at A and 

 a source of variable alternating potential at V. The alternating poten- 

 tial V could be applied or omitted by closing or opening the switch at 

 T. A small heating coil was wound on the rod C, and another similar 

 heating coil was wound on a second copper rod E placed immediately 

 below the contact of C with M. 



Figure 12. Apparatus for compar- 

 ison of rectified current with thermal 

 current. 



