2X8 Mr. A. T. Waterman on the Positive 



Strips are quite easily prepared for use. But it is not readily 

 soldered. For this reason the method of mounting was 

 changed somewhat, and two small brass screw clamps were 

 employed to hold the ends of the strip. It was difficult to 

 obtain a uniform contact for all variations in temperature, 

 as on account of the softness of the mineral too much pressure 

 could not be applied at the clamps without crushing the strip 

 to a powder, whereas too little pressure meant serious insta- 

 bility of resistance. The surface of the strip was flush with 

 the lower plate of the apparatus for measuring ejm when 

 cool, but no arrangement was used to make certain that it 

 was thus when hot. The thickness of the strips was about 

 1'5 mm., somewhat greater than that of the Pt strips used, 

 otherwise the dimensions were practically the same. 



The electrical conductivity of molybdenite presents peculiar 

 and interesting features. With small applied potential 

 differences the resistance of the strips is very high, but 

 diminishes rapidly with increasing potential differences. 

 No doubt this alteration in conductivity is partly due to the 

 rise in temperature caused by the electric current, and is 

 similar to the effects shown by a large number of minerals 

 which have been examined in detail by Kcenigsberger and 

 Schilling *. The effect does not, however, appear to be 

 entirely, or perhaps even principally, a temperature effect, 

 since it is shown under very low voltages when the currents 

 are too small to cause any perceptible increase in the tem- 

 perature of the material. Thus with one strip the resistance 

 was 8*1 x 10 5 ohms for 2 volts applied potential difference 

 and 3*52 X 10 6 ohms for 01 volt. The diminution of resist- 

 ance with increasing potential difference continues uniformly 

 up to a certain potential difference, which varies to some 

 extent with different specimens and according to the treat- 

 ment to which they have been subjected ; at this point, which 

 will be referred to as " the break/' the resistance diminishes 

 suddenly and the mineral couducts, at a red heat, with a 

 comparatively low resistance. Before the voltage has been 

 raised sufficiently for the break to occur the resistance 

 diminishes, as has been noted above, with increasing voltage, 

 whereas after the break the resistance diminishes with 

 diminishing voltage. This difference may be regarded as 

 characteristic of two states, a and ft, of the material which 

 occur before and after the break. The phenomenon of the 

 break is reversible, but the rate of change of state decreases 

 after long heating. The substance also tends to persist in 

 the state in which it was when the current was turned off. 

 * Ann. der Phys. vol. xxxii. p. 179 (1910). 



