378 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



Molybdenite is also an extremely sensitive detector for electric 

 waves in wireless telegraphy, and may also be employed in experi- 

 ments on telephony and in many other experiments where it is required 

 to measure small electric oscillations. 



The manner of mounting and employing the substance is substan- 

 tially the same in these several applications, and is capable of several 

 variations, only one of which will be given here. Molybdenite, M0S2, 

 is a mineral occurring in nature in the form of hexagonal prisms with 

 eminent cleavage parallel to the base, and may be scaled off in thin 

 sheets, a few sq. cm. in area, resembling bits of tin-foil. In the present 

 experiments a thin sheet so obtained was mounted in the manner 

 shown in the sectional drawing of Figure 1. 



Figure 1. — Rectifier. 



A thin, circular piece of molybdenite ^ (M, Figure 1), about 1 sq. cm. 

 in area, is clamped tightly between a piece of mica N and the hollow 

 brass post A, by means of a brass cap C screwed down on the post A. 

 The molybdenite is thus held in electrical connection with the annular 

 surface of the end of the hollow brass post A, which is in turn metal- 

 lically connected with the binding post G. Separated from A by an 

 air space, a small pointed brass rod B is screwed up through a metallic 

 strip H attached to a second binding post F. The binding posts and 

 the holder for the molybdenite are rigidly supported by a porcelain 

 base PP. The seat of the action of the molybdenite as a rectifier is 

 at the small region of contact between the molybdenite and the pointed 

 rod. In the construction of the rectifier this contact is adjusted by 

 screwing the rod up through H until a galvanometer in series with 

 the device and a soui'ce of alternating voltage (of about .05 volt) gives 



^ Molybdenite free from iron should be used. 



