318 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



dum, it occurred to the writer that a further study of the electrical be- 

 havior of this substance would be interesting. In the course of this 

 study, an account of which has been published in the Physical Review 2 

 for July, 1907, it was discovered that when a piece of carborundum is 

 placed in a clamp between contact electrodes, the heterogeneous con- 

 ductor consisting of the carborundum and the electrodes permits the 

 passage of a greater current in one direction than in the reverse di- 

 rection under the same applied voltage. The device can be used as a 

 rectifier for small alternating currents and oscillations. The phenom- 

 enon is very striking. For example, with one specimen under an elec- 

 tromotive force of 30 volts the current in one direction is 4000 times 

 as great as the current in the opposite direction under the same 

 external voltage. 



Although the rectified current is not large (in the case j ust cited, 3 

 milliamperes in one direction and .00075 milliamperes in the opposite 

 direction) such a rectifier, being constructed entirely of solid parts, 

 possesses sufficient permanence and constancy to permit of many use- 

 ful applications, where the detection and measurement of small alternat- 

 ing currents is required. As an example of such applications details 

 are given in Part I of the employment of the rectifier in the construc- 

 tion of an alternating current voltmeter operable with an extremely 

 small consumption of energy. 



Questions arising in Connection with the Phenomenon. — Many ques- 

 tions of theoretical interest arise in connection with the phenomenon. 

 Is the action localized at the surface of contact between the crys- 

 tal and the metallic electrode ? Is the action due to electrolytic 

 polarization ? Is the action thermoelectric, conditioned on unequal 

 heating of the two electrode contacts 1 If the phenomenon is novel, 

 how is it related to the hitherto studied properties of conductors ? 



In the experiments on carborundum performed by the writer the in- 

 vestigation of these questions met with limitations on account of the 

 form of occurrence of the carborundum in discrete masses to which 

 electrodes could not be rigidly attached ; so that the conditions at the 

 electrodes could not be widely varied. However, by increasing the 

 pressure of the electrodes against the carborundum beyond a certain 

 limit, and by cathodically platinizing the surfaces of the carborundum 

 at both the contact areas, the rectification, though not entirely elimin- 

 ated, was rendered very imperfect ; that is to say, the ratio of the 

 strength of the current in one direction to that in the reverse direction 

 approached unity. On the other hand, platinizing one only of the 



2 Pierce : Physical Review, 25, 31-60 (1907). 



