322 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



ease with which the thermoelectric property of the substance may he 

 studied, admirably adapts it to the present experiments. 



The Molybdenite Rectifier. — The rectifying action of the molybde- 

 nite was first obtained with a thin, flat specimen of the mineral held 

 between flat contact electrodes in a clamp of which the two jaws were 

 insulated from one another. With this form of mounting the molyb- 

 denite also acts as a receiver for electric waves with or without a battery 

 in the local circuit. 



It was soon found, however, that the apparatus was more sensitive 

 as a receiver for electric waves and as a rectifier, when one of the con- 

 tacts between the molybdenite and the 

 electrode had a high resistance. A form 

 of mounting in which this is attained 

 is shown in section in Figure 2. T is 

 a threaded brass post on the top of 

 which is placed a disc of mica, N. On 

 top of the mica is a thin circular disc 

 of the molybdenite M, with an area of 

 about 1 square centimeter, leaving a 

 projection of the mica beyond the pe- 

 riphery of the molybdenite. A hollow 

 cap, D, threaded inside and having a 

 conical hole at the top, is screwed down 

 on the post T so as to clamp the 

 molybdenite between the mica disc 7 

 and the annular shoulder of the cap, 

 with the upper surface of the molyb- 

 denite exposed above. At the free 

 surface of the molybdenite contact is 

 made with the metallic rod P. 8 

 The rod P was either supported unadjustably, as in the author's ex- 



FlGCRE 2. 



Holder for molyb- 

 denite. 



7 Tlie purpose of the mica disc under the molybdenite is to confine the current 

 as much as possible to the upper layer of the molybdenite. This was done so as 

 not to complicate the phenomenon by conduction across the laminae of the sub- 

 stance, and also so that when the detector is immersed in oil in some of the later 

 experiments, the oil shall have free play over the conducting surface and over the 

 contacts, and serve the better to avoid possible changes of temperature of the 

 essential parts of the apparatus. 



8 In the diagrams of Figure 2 and Figure 3 the lower end of the rod P is shown 

 pointed. It is found, however, that the end of the rod P may be blunt or even 

 flat with an area as great as 4 sq. mm. without much loss of sensitiveness of the 

 instrument as a receiver for electric waves or as a rectifier. 



