PIERCE. — CRYSTAL RECTIFIERS FOR ELECTRIC CURRENTS. 321 



30 40 60 



CENTIVOLTS 



Figure 1. 



Current-voltage curves for anatase, 

 with direct current. 



the lower curve was with the current in the opposite direction, as in- 

 dicated by the arrows. The contact pressure in this experiment was 2 

 kilograms. These curves have the same general form as those obtained 

 in the experiments on car- 

 borundum. By a compari- 

 son with Part I, it is seen, 

 however, that the anatase 

 gives much larger cur- 

 rents with a small applied 

 voltage than does the 

 carborundum. This char- 

 acterizes the anatase as 

 a much more sensitive 

 rectifier for small alternat- 

 ing voltages and as a much 

 more sensitive detector for 

 electric waves than is the 

 carborundum. 



Brookite. — This is an- 

 other crystal form of Ti0 2 , 

 which was found to serve as a rectifier of small alternating currents 

 with about the same sensitiveness as anatase. Although a considerable 

 amount of time was spent in experimenting with anatase and brookite, 

 these substances, occurring like carborundum in discrete pieces to which 

 terminals could not be attached, did not serve to throw much light on 

 the phenomenon. Numerical data in regard to them are, therefore, 

 omitted. 



Molybdenite. 



One of the most sensitive of the rectifiers thus far investigated makes 

 use of molybdenite as a member. 6 Molybdenite, with the chemical 

 formula MoS 2 , is a mineral occurring in nature in the form of tabular 

 hexagonal prisms with eminent cleavage parallel to the base of the 

 prism. The cleavage of the crystal resembles that of mica, and thin 

 sheets of the mineral several square centimeters in area may be scaled 

 off from a large crystal of molybdenite. These sheets have a metallic 

 lustre and look not unlike sheets of lead foil. They can be readily 

 electroplated with copper, so that connecting wires may be soldered to 

 them. This property, together with the thinness of the sheets and the 



6 See also G. W. Pierce : A simple Method of Measuring the Intensity of 

 Sound, These proceedings, 43, 377 (Feb., 1908), in which the Molybdenite Kec- 

 tifier was employed. 

 vol. xliv- — 21 



